Drug action: pharmaceutic, pharmacokinetic, and pharmacodynamic phases Exams

Drug action: pharmaceutic, pharmacokinetic, and pharmacodynamic phases

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Study Questions

Practice Exercise

Question 1 :

Practice question 1:

Pharmacology: A patient-Centered Nursing Process Approach – Key, Hayes & McCuistion

Scoring rule: 0/1

Total Marks: 1

The nurse anticipates that the health care provider will order which laboratory test for an older adult with renal dysfunction?

A. Creatinine clearance B. Glomerular filtration rate C. Urine specific gravity D. Urine pH

Explanation

Renal function assessment in older adults requires the most accurate measure of kidney filtration capacity. Creatinine clearance testing is the gold standard for estimating glomerular filtration rate (GFR) and identifying renal impairment. Since muscle mass decreases with age, serum creatinine alone may appear normal despite reduced kidney function, making creatinine clearance especially important.

Rationale for correct answer:

A. Creatinine clearance – This test measures the rate at which creatinine is cleared from the blood by the kidneys and closely reflects the GFR. In older adults with reduced muscle mass, it provides a more accurate indicator of renal function than serum creatinine alone.

Rationale for incorrect answers:

B. Glomerular filtration rate – While GFR is a key measure of kidney function, it is often estimated based on creatinine clearance rather than measured directly in routine practice. Direct creatinine clearance testing is more precise in this setting.

C. Urine specific gravity – This measures urine concentration and hydration status but does not accurately evaluate overall renal function or filtration capacity.

D. Urine pH – This indicates the acidity or alkalinity of urine, which can help in diagnosing some metabolic or urinary disorders, but it does not reflect kidney filtration efficiency.

Take-home points:

Creatinine clearance is the most accurate test for assessing renal function in older adults.
Serum creatinine may be misleading in elderly patients due to reduced muscle mass.
Kidney filtration assessment is essential before prescribing drugs excreted by the kidneys.

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Question 2 :

Practice question 2:
Pharmacology: A patient-Centered Nursing Process Approach – Key, Hayes & McCuistion
Scoring rule: 0/1
Total Marks: 1

A student nurse is studying the phases of drug action. Which statement by the student indicates to the nursing instructor that the student understands the pharmaceutic phase?

A. “To achieve drug action, drugs are moved by four processes.” B. “For the drug to cross the biologic membrane, the drug becomes a solution.” C. “In this phase, drugs are concentrated and a biologic or physiologic response occurs.” D. “The pharmaceutic phase is the process by which the drug becomes available to body fluids and tissue.”

Explanation

The pharmaceutic phase refers to the initial stage of drug action for oral medications, where the drug is disintegrated and dissolved so it can be absorbed into body fluids and tissues. This phase is unique to oral dosage forms and precedes the pharmacokinetic and pharmacodynamic phases.

Rationale for correct answer:

D. “The pharmaceutic phase is the process by which the drug becomes available to body fluids and tissue.” – This is correct because it defines the phase as the breakdown and dissolution of the drug so it can enter circulation for absorption.

Rationale for incorrect answers:

A. “To achieve drug action, drugs are moved by four processes.” – This describes the pharmacokinetic phase, which involves absorption, distribution, metabolism, and excretion, not the pharmaceutic phase.

B. “For the drug to cross the biologic membrane, the drug becomes a solution.” – While dissolution into a solution is part of the pharmaceutic phase, this statement focuses only on one step and does not capture the full definition of the phase.

C. “In this phase, drugs are concentrated and a biologic or physiologic response occurs.” – This describes the pharmacodynamic phase, where the drug interacts with receptors to produce effects, not the pharmaceutic phase.

Take-home points:

The pharmaceutic phase occurs only with oral medications.
It involves disintegration and dissolution before absorption.
It precedes the pharmacokinetic and pharmacodynamic phases.

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Question 3 :

Practice question 3:

RN-pharmacology-for-nursing – ATI

Scoring rule: 0/1

Total Marks: 1

A client is prescribed phenobarbital sodium (Luminal) for a seizure disorder. The medication has a long half-life of 4 days. Based on this half-life, the medication will most likely be prescribed

A. once a day. B. twice a day. C. three times a day. D. four times a day.

Explanation

Phenobarbital sodium has a long half-life of approximately 4 days, meaning it remains in the body for an extended period before being eliminated. Drugs with long half-lives generally require less frequent dosing to maintain therapeutic blood levels while avoiding drug accumulation and toxicity. This allows for once-daily administration in most cases, which can also improve medication adherence.

Rationale for correct answer:

A. Once a day – The long half-life means the drug stays in circulation for several days, so once-daily dosing is sufficient to maintain stable plasma concentrations and control seizures

Rationale for incorrect answers:

B. Twice a day – Unnecessary for a drug with such a prolonged half-life, as this could lead to excessive drug accumulation and an increased risk of sedation or toxicity.

C. Three times a day – Appropriate for short-acting medications but not for long-acting barbiturates like phenobarbital.

D. Four times a day – This frequent dosing is used for drugs with very short half-lives; with phenobarbital, it would cause dangerously high plasma drug levels and adverse effects.

Take-home points:

Long half-life drugs require less frequent dosing to maintain therapeutic levels.
Phenobarbital can be given once daily for seizure control due to its prolonged duration of action.
More frequent dosing of long-acting drugs increases the risk of toxicity without improving efficacy.

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Question 4 :

Practice question 4:

RN-pharmacology-for-nursing – ATI

Scoring rule: +/-

Total Marks: 1

A nurse educator is reviewing medication dosages and factors that influence medication metabolism with a group of nurses. Medication dosages may need to be decreased for which of the following reasons? Select all that apply

A. Increased renal excretion B. Increased medication-metabolizing enzymes C. Liver failure D. Peripheral vascular disease E. Concurrent use of medication metabolized by the same pathway

Explanation

Medication metabolism is primarily carried out in the liver, and any condition or interaction that reduces the liver’s ability to break down drugs can lead to increased drug levels in the body. In such cases, the dosage must be decreased to avoid toxicity. Additionally, when two drugs are metabolized through the same enzymatic pathway, they compete for metabolism, slowing the clearance of one or both medications.

Rationale for correct answers:

3. Liver failure – Reduced liver function decreases metabolism, causing drugs to stay in the system longer, requiring lower dosages to prevent toxicity.

5. Concurrent use of medication metabolized by the same pathway – Competition for the same metabolic enzymes slows drug clearance, increasing drug levels and requiring dosage reduction.

Rationale for incorrect answers:

1. Increased renal excretion – This speeds up drug elimination through the kidneys, potentially requiring an increased, not decreased, dosage to maintain therapeutic effects.

2. Increased medication-metabolizing enzymes – This accelerates metabolism, leading to lower drug levels; dosages may need to be increased rather than decreased.

4. Peripheral vascular disease – This may impair drug delivery to tissues but does not directly impact drug metabolism in the liver.

Take-home points:

Liver failure reduces metabolism, increasing the risk of toxicity.
Drug-drug metabolic competition can slow clearance and raise blood concentrations.
Dosage adjustments are based on how metabolism and elimination are affected, not just the presence of disease.

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Question 5 :

Practice question 5:

Scoring rule: 0/1

Total Marks: 1

A client with advanced liver disease is prescribed a drug with high first-pass metabolism. Which route is most appropriate to avoid reduced bioavailability?

A. Oral B. Sublingual C. Rectal D. Sustained-release oral tablet

Explanation

Drugs with high first-pass metabolism undergo extensive breakdown in the liver before reaching systemic circulation when given orally. In a client with advanced liver disease, this effect is even more pronounced, greatly reducing bioavailability. Administering the drug sublingually allows it to be absorbed directly into the systemic circulation via the oral mucosa, bypassing the liver initially and maintaining therapeutic levels.

Rationale for correct answer:

B. Sublingual – Absorption under the tongue delivers the drug directly into systemic circulation, bypassing the liver’s first-pass effect. This route is especially useful for drugs with high hepatic metabolism or in patients with compromised liver function. It allows for faster onset and more predictable plasma drug concentrations.

Rationale for incorrect answers:

A. Oral – Drugs taken orally are absorbed through the GI tract and pass through the liver via the portal circulation, undergoing extensive first-pass metabolism, which reduces bioavailability. This is not ideal for patients with significant liver impairment.

C. Rectal – While partial absorption from the lower rectum can bypass the liver, a significant portion of rectal blood flow still goes through the portal vein, meaning some first-pass metabolism still occurs.

D. Sustained-release oral tablet – Although it provides slow, steady drug release, it is still absorbed in the GI tract and subject to the first-pass effect, making it unsuitable for avoiding reduced bioavailability in this scenario.

Take-home points:

Sublingual administration bypasses the liver and avoids first-pass metabolism.
High first-pass metabolism drugs have reduced oral bioavailability, especially in liver disease.
Drug route selection can significantly impact absorption, onset, and therapeutic effect.

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Question 6 :

Practice question 6:

Scoring rule: 0/1

Total Marks: 1

A nurse should advise a client not to crush an enteric-coated tablet primarily because:

A. It may cause premature dissolution and alter absorption site. B. It reduces the drug’s potency, decreasing risk of toxicity. C. It ensures the drug is dissolved only in acidic pH environments. D. It eliminates the possibility of delayed absorption.

Explanation

Enteric-coated tablets are designed with a special coating that allows them to pass through the stomach intact and dissolve in the alkaline environment of the small intestine. Crushing the tablet destroys this coating, leading to premature drug release in the stomach, which can result in reduced effectiveness, irritation of the gastric mucosa, or altered absorption patterns.

Rationale for correct answer:

A. It may cause premature dissolution and alter absorption site – The coating protects the drug from stomach acid and protects the stomach from potential irritation. Crushing disrupts this process, causing drug release in the wrong part of the GI tract, altering its therapeutic effect and increasing the risk of gastric irritation.

Rationale for incorrect answers:

B. It reduces the drug’s potency, decreasing risk of toxicity – Crushing does not inherently reduce drug potency; instead, it can lead to faster absorption and possibly increased toxicity if the entire dose is released at once.

C. It ensures the drug is dissolved only in acidic pH environments – Enteric coatings are made to dissolve in alkaline, not acidic, environments. This statement reflects a misunderstanding of the drug’s design.

D. It eliminates the possibility of delayed absorption – While crushing can speed absorption, the primary concern is loss of targeted release and site-specific absorption, not just timing.

Take-home points:

Enteric coating protects both the drug from stomach acid and the stomach from irritation.
Crushing can cause premature drug release and change absorption location.
Always check drug formulation before altering its form to avoid safety and efficacy issues.

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Question 7 :

Practice question 7:

Scoring rule: 0/1

Total Marks: 1

A nurse is teaching a student nurse about bioavailability. Which of the following statements about bioavailability is most accurate?

A. It refers only to the rate of GI drug dissolution. B. Intravenous drugs have 100% bioavailability. C. Oral drugs generally have bioavailability close to 100%. D. The first-pass effect increases oral bioavailability.

Explanation

Bioavailability refers to the percentage of an administered drug that reaches the systemic circulation in its active form. For intravenous (IV) administration, the drug enters directly into the bloodstream without absorption barriers or first-pass metabolism, making its bioavailability 100%. This contrasts with oral drugs, which undergo variable absorption and hepatic metabolism before reaching systemic circulation.

Rationale for correct answer:

B. Intravenous drugs have 100% bioavailability – IV drugs bypass absorption barriers and the first-pass effect, delivering the entire administered dose into circulation, resulting in complete systemic availability.

Rationale for incorrect answers:

A. It refers only to the rate of GI drug dissolution – Bioavailability involves both the rate and extent of absorption into systemic circulation, not just GI dissolution.

C. Oral drugs generally have bioavailability close to 100% – Most oral drugs have less than 100% bioavailability due to incomplete absorption and first-pass metabolism in the liver.

D. The first-pass effect increases oral bioavailability – The first-pass effect decreases oral bioavailability because a portion of the drug is metabolized in the liver before reaching systemic circulation.

Take-home points:

IV administration provides 100% bioavailability by directly entering the bloodstream.
Oral bioavailability is reduced by absorption barriers and first-pass metabolism.
Bioavailability measures both rate and extent of drug absorption into systemic circulation.

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Question 8 :

Practice question 8:

Scoring rule: 0/1

Total Marks: 1

In a client with low albumin levels, what is the likely effect on a highly protein-bound drug?

A. Decreased drug effect due to more binding B. Increased free drug levels and toxicity risk C. Accelerated liver metabolism D. Delayed renal elimination

Explanation

Albumin is the primary plasma protein that binds many drugs in the bloodstream. Highly protein-bound drugs rely on albumin for transport and controlled release. In a client with low albumin levels (e.g., due to liver disease, malnutrition, or chronic illness), fewer binding sites are available, resulting in more unbound or “free” drug in circulation

Rationale for correct answer:

B. Increased free drug levels and toxicity risk – With reduced protein binding, the free active form of the drug rises, enhancing therapeutic and potentially toxic effects. This is particularly important for drugs with narrow therapeutic ranges.

Rationale for incorrect answers:

A. Decreased drug effect due to more binding – Low albumin leads to less binding, not more. This would increase, not decrease, the drug’s effect.

C. Accelerated liver metabolism – Albumin levels do not directly influence liver metabolism. The liver processes drugs based on enzyme activity, not plasma protein levels.

D. Delayed renal elimination – While protein binding can affect filtration, low albumin primarily impacts drug distribution, not elimination rate.

Take-home points:

Low albumin increases the free fraction of protein-bound drugs.
Higher free drug levels raise the risk of toxicity, especially in drugs with narrow therapeutic indices.
Monitoring plasma levels is crucial when administering highly protein-bound drugs in hypo-albuminemic clients.

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Practice Exercise 2

Question 9 :

Practice question 9:

Pharmacology: A patient-Centered Nursing Process Approach – Key, Hayes & McCuistion

Scoring rule: 0/1

Total Marks: 1

The nurse is caring for a patient with congestive heart failure who is receiving digoxin (Digitek, Lanoxicaps, Lanoxin). The nurse plans to take which action when administering digoxin?

A. Check the peak levels of digoxin. B. Check the trough levels digoxin. C. Monitor for tachyphylaxis. D. Monitor the therapeutic range of digoxin.

Explanation

Digoxin has a narrow therapeutic range (typically 0.5–2 ng/mL), meaning small changes in drug concentration can lead to toxicity or subtherapeutic effects. In patients with congestive heart failure, maintaining the drug within this range is essential for maximizing cardiac benefits while preventing adverse effects such as arrhythmias, visual disturbances, and gastrointestinal upset.

Rationale for correct answer:

D. Monitor the therapeutic range of digoxin – Monitoring serum levels within 0.5–2 ng/mL ensures the drug remains effective without causing toxicity, which is especially important given its narrow safety margin.

Rationale for incorrect answers:

A. Check the peak levels of digoxin – Peak levels are not routinely used for digoxin monitoring, as they do not reliably indicate toxicity risk due to the drug’s tissue binding and long distribution phase.

B. Check the trough levels of digoxin – While trough levels (just before the next dose) are more useful than peaks, the primary focus is on maintaining overall therapeutic serum concentration rather than monitoring troughs alone.

C. Monitor for tachyphylaxis – Tachyphylaxis refers to rapid drug tolerance development; digoxin does not typically cause this.

Take-home points:

Digoxin requires close monitoring due to its narrow therapeutic index.
The safe serum range is generally 0.5–2 ng/mL.
Signs of toxicity include arrhythmias, visual changes (yellow/green halos), and GI upset.

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Question 10 :

Practice question 10:

Pharmacology: A patient-Centered Nursing Process Approach – Key, Hayes & McCuistion

Scoring rule: 0/1

Total Marks: 1

When reviewing the client’s medication regimen, the nurse understands that the interval of drug dosage is related to what?

A. Half-life B. Stimulation of receptors C. Therapeutic index D. Trough level

Explanation

The half-life of a drug is the time required for half of the drug concentration in the body to be eliminated. This pharmacokinetic property determines how often a medication should be administered to maintain therapeutic levels without causing toxicity. A drug with a short half-life requires more frequent dosing, whereas a drug with a long half-life can be given less often while still maintaining effective plasma concentrations.

Rationale for correct answer:
A. Half-life – The dosing interval is directly influenced by the drug’s half-life, ensuring a steady therapeutic level and avoiding fluctuations that could cause subtherapeutic effects or toxicity.

Rationale for incorrect answers:

B. Stimulation of receptors – This describes a mechanism of action, not the factor that determines dosing frequency.

C. Therapeutic index – While it indicates the safety margin of a drug, it does not directly dictate the dosing interval; instead, it guides safe dosing limits.

D. Trough level – Trough levels help determine if a drug remains in the therapeutic range before the next dose but are a monitoring tool, not the primary determinant of the dosing schedule.

Take-home points:

• Half-life is the key factor in determining dosing frequency.

• Short half-life drugs require more frequent administration than long half-life drugs.

• Understanding half-life helps balance drug effectiveness and safety.

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End of Questions

Exams on Drug action: pharmaceutic, pharmacokinetic, and pharmacodynamic phases

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Lessons

Table of Contents

Objectives
Introduction
Pharmaceutic phase
Pharmacokinetic phase
Practice Exercise
Pharmacodynamic Phase
Additional Concepts And Clinical Applications
Summary
Practice Exercise 2
Comprehensive Questions

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Objectives

Differentiate the three phases of drug action (pharmaceutic, pharmacokinetic, pharmacodynamic) and their clinical relevance.
Explain how drug absorption is influenced by route, formulation, and physiological factors.
Understand the role of protein binding and distribution in drug effectiveness and toxicity.
Describe drug metabolism, excretion, and the impact of liver or kidney dysfunction.
Apply receptor theory to explain drug actions, including agonists and antagonists.
Interpret dose-response relationships, therapeutic index, and drug safety margins.
Identify onset, peak, and duration of drug action and their importance in dosing.
Recognize adverse effects, tolerance, pharmacogenetics, and individual variability in drug response.

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Introduction

Understanding drug action involves three fundamental and sequential phases: the pharmaceutic, pharmacokinetic, and pharmacodynamic phases.

These phases describe how a drug is transformed from a dosage form into an active entity in solution, how the body handles that drug (ADME—absorption, distribution, metabolism, excretion), and how the drug produces a biologic or physiologic response.

The pharmaceutic phase applies only to oral solid dosage forms (tablets, capsules) and includes disintegration and dissolution so the drug can cross biologic membranes. Parenteral, sublingual, transdermal, inhaled, and many topical routes bypass this phase.

The pharmacokinetic phase (ADME) determines onset, intensity, duration, and elimination of drug effect.

The pharmacodynamic phase explains how a drug produces a response at receptor or effector sites, and why therapeutic and adverse effects occur.

Clinical relevance / pathophysiology links: organ dysfunction (GI disease, liver disease, kidney disease), age-related physiologic changes, and genetic differences alter one or more phases and therefore change dosing, onset, duration, and toxicity risk.

Understanding each phase allows safe medication administration, appropriate monitoring, patient education, and early recognition of altered drug responses.

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Pharmaceutic phase

(Only for oral solid dosage forms — tablets and capsules)

Definition & overview
• The pharmaceutic phase is the process by which a solid oral dosage form disintegrates into smaller particles and then dissolves (dissolution) into GI fluids so the drug is in solution and available for absorption across biologic membranes. Liquid forms (solutions, syrups) are already in solution and bypass disintegration.
• Approximately 80% of drugs are taken by mouth; therefore pharmaceutic-phase processes are highly relevant to clinical nursing.

Key processes
• Disintegration — mechanical/chemical breakdown of a tablet/capsule into smaller particles.
• Dissolution — the subsequent dissolving of those particles into GI fluid to form a solution that can cross membranes.

Formulation factors that influence the pharmaceutic phase
• Excipients / fillers / binders / disintegrants — inert ingredients added to achieve proper size, shape, stability, or to enhance dissolution (e.g., some salts increase solubility).
• Particle size — smaller particles have greater surface area and dissolve faster.
• Coatings / special formulations — enteric-coated tablets resist stomach acid and dissolve in the more alkaline small intestine; sustained- / extended-release formulations release drug slowly over time. Crushing or splitting these preparations alters intended release and can cause toxicity or therapeutic failure.

• Liquid vs solid forms — liquids are generally absorbed faster because they bypass disintegration and dissolution.

Physiologic & pathophysiologic influences
• Gastric pH: the stomach’s acidity (pH 1–2) promotes disintegration/dissolution of many drugs; alkaline environments may ionize some drugs and reduce membrane crossing. Infants and older adults often have less gastric acidity (hypochlorhydria) → slower dissolution and absorption of drugs normally absorbed in the stomach.
• Gastric motility & emptying time: delayed emptying (gastroparesis, opioids, postoperative ileus) prolongs contact of drug with stomach, delaying arrival to small intestine (where many drugs are absorbed). Rapid emptying shortens contact time.
• Food & fluid: food can delay, enhance, or reduce dissolution depending on the drug (e.g., fat-rich meals slow gastric emptying; some drugs cause gastric irritation and require administration with food).
• Surgical changes: resections (partial gastrectomy, bowel resection) reduce surface area, alter pH, and substantially change dissolution/absorption dynamics.

Examples & mechanistic notes
• Penicillin salts: penicillin G is acid-labile and poorly absorbed in gastric acid; forming sodium or potassium salts increases absorbability and mitigates gastric destruction.
• Enteric-coated tablets: can remain in stomach for prolonged periods; onset may be delayed if gastric emptying is slow.

Nursing insights
• Determine whether a tablet/capsule may be crushed or split — consult drug references or pharmacy before altering dosage forms. Never crush enteric-coated, sustained-release, or otherwise modified-release products unless explicitly allowed.
• Assess swallowing safety; consider alternative formulations (liquid, sublingual, transdermal, parenteral) for patients with dysphagia.
• Take medication administration times and meal instructions seriously — provide education about why certain drugs are taken with or without food.
• Be alert for patient factors that delay dissolution (PPI therapy, antacids, bariatric surgery, gastroparesis) and consider route changes when indicated.

Take Practice Test on Pharmaceutic phase

Pharmacokinetic phase

(What the body does to the drug — ADME)

Overview
Pharmacokinetics consists of four interrelated processes: Absorption, Distribution, Metabolism (biotransformation), and Excretion (elimination). These processes determine the concentration-time profile of a drug in the body and therefore its clinical effects.

ABSORPTION

Definition & mechanisms
• Absorption is the movement of drug from the site of administration into the bloodstream (systemic circulation). Mechanisms include:

Passive diffusion (most common; drug moves down a concentration gradient without energy),
Active transport (requires carrier proteins and energy to move against gradients),
Pinocytosis (cellular engulfment of drug particles).

Primary absorption sites & routes
• Oral drugs: primarily absorbed in the small intestine across mucosal villi (large surface area).
• Parenteral: IV (direct into bloodstream; 100% bioavailability), IM and subcutaneous (absorption rate depends on blood flow and tissue vascularity).
• Other: sublingual, transdermal, inhaled, topical, rectal — each route has unique absorption kinetics and bypasses or partially bypasses first-pass liver metabolism.

Factors influencing absorption
• Drug properties: lipid solubility (lipid-soluble drugs cross membranes faster), particle size, ionization (nonionized are absorbed more readily).
• pH & ionization: acidic drugs are less ionized in acidic environments (e.g., aspirin in stomach), facilitating absorption there; basic drugs absorb better in more alkaline environments.
• Blood flow: perfusion at the absorption site (e.g., shock, vasoconstriction, local edema) decreases absorption.
• Surface area & integrity: loss of mucosal villi (celiac disease, extensive resections) or mucosal disease reduces absorption.
• First-pass effect: after absorption from the GI tract, drugs reach the liver via portal circulation and may be partially metabolized before reaching systemic circulation, reducing bioavailability. Examples of drugs with extensive first-pass metabolism include certain nitrates and many analgesics; such drugs may require higher oral doses or alternate routes.
• Co-administration factors: food, other drugs (antacids, enzyme inducers/inhibitors), exercise, and physiologic states (fasting, stress) alter absorption.

Clinical & pathophysiologic links
• Infants: higher gastric pH → greater absorption of acid-labile drugs (e.g., increased penicillin absorption).
• Older adults: reduced GI motility and lower gastric acid can slow absorption of some drugs.
• Shock/hypoperfusion: reduced splanchnic blood flow delays oral absorption and may shift reliance to parenteral routes.
• IM injection site: muscles with greater blood flow (deltoid) absorb faster than less vascular sites (gluteus).

Bioavailability
• Bioavailability = percentage of an administered dose that reaches systemic circulation as active drug. IV = 100%; oral bioavailability is typically <100% due to incomplete absorption and first-pass metabolism. Factors altering bioavailability: formulation, route, GI mucosa/motility, presence of food/other drugs, hepatic function.

Nursing insights
• Select route appropriate to clinical condition (e.g., avoid oral if rapid effect needed or GI absorption impaired).
• Follow administration instructions regarding food and fluids, and educate patients.
• Monitor for delayed or reduced effect in conditions that alter perfusion or GI integrity.
• Recognize that co-prescribed drugs (antacids, PPIs, enzyme inducers/inhibitors) may alter absorption and bioavailability.

DISTRIBUTION

Definition & core determinants
• Distribution refers to the reversible transfer of a drug between the bloodstream and tissues. Key determinants include blood flow to tissues, capillary permeability, tissue affinity for the drug, and plasma protein binding.

Volume of distribution (Vd)
• Vd is a theoretical concept describing how a drug distributes between plasma and the rest of the body; drugs with large Vd often reside in tissues and may have prolonged half-lives.

Protein binding
• Many drugs bind to plasma proteins (primarily albumin). Bound drug is pharmacologically inactive; only the free (unbound) fraction is available to cross membranes, interact with receptors, and be eliminated.
• Ranges (common classification used clinically):

High protein binding: >89% bound
Moderately high: 61–89%
Moderate: 30–60%
Low: <30%
• Clinical consequences: When two highly protein-bound drugs are coadministered, they compete for binding sites → increased free drug fraction → risk of toxicity. Low albumin states (malnutrition, liver disease, burns, nephrotic syndrome) decrease binding capacity → increased free drug → potential toxicity.

Barriers & tissue sequestration
• Blood–brain barrier (BBB): tight junctions and lipid-rich cell membranes restrict passage; lipid-soluble, unbound drugs cross most readily. Inflammatory states (meningitis) can disrupt BBB and allow entry of normally excluded drugs.
• Placental barrier: many drugs cross and can affect the fetus; both lipid-soluble and some water-soluble drugs cross the placenta—risk-benefit must be carefully weighed in pregnancy.
• Breast milk: drugs may be secreted and affect nursing infants; knowledge of lactation compatibility is necessary.
• Tissue accumulation: some drugs accumulate in fat (lipophilic drugs), bone, liver, muscle, or ocular tissues, which can create depots that prolong effect and complicate toxicity.

Distribution impediments
• Abscesses & exudates: minimal blood supply and acidic environment reduce antibiotic penetration; drainage and local management may be required to achieve therapeutic effect.
• Tumors and poorly perfused tissues may receive inadequate drug levels.

Nursing insights
• Assess nutritional status, albumin levels, and conditions affecting protein binding.
• Anticipate altered dosing in hypoalbuminemia and with polypharmacy involving highly protein-bound drugs.
• Consider tissue sequestration when planning duration and monitoring for toxicity.
• Be cautious with CNS-active drugs in patients with disrupted BBB.
• Review pregnancy and lactation status before prescribing potentially teratogenic or excretable drugs.

METABOLISM (BIOTRANSFORMATION)

Purpose & major site
• Metabolism converts drugs (often lipophilic) into more polar, water-soluble metabolites that can be readily excreted, predominantly by the liver using enzyme systems such as cytochrome P450 (CYP) families. Metabolism may inactivate a drug, produce an active metabolite, or convert prodrugs into active forms.

Phases of metabolism
• Phase I (functionalization reactions): oxidation, reduction, hydrolysis — frequently mediated by CYP enzymes.
• Phase II (conjugation reactions): glucuronidation, sulfation, acetylation — attach polar groups to increase water solubility.

Genetic & physiologic variability
• Pharmacogenetics: genetic polymorphisms in metabolic enzymes produce fast or slow metabolizers for particular drugs (e.g., CYP2D6, CYP3A4 variants) and can dramatically affect drug levels and responses.
• Age: neonates/infants have immature hepatic enzyme systems; older adults often have reduced hepatic blood flow and enzyme activity.
• Disease: hepatic disease (cirrhosis, hepatitis) reduces metabolic capacity, prolongs drug half-life, and increases toxicity risk.
• Drug–drug interactions: enzyme inhibitors (decrease metabolism → increased levels/toxicity) and inducers (increase metabolism → decreased levels/therapeutic failure).

First-pass (hepatic) effect
• Drugs absorbed from the gastrointestinal tract pass through the portal circulation to the liver where a significant fraction may be metabolized before reaching systemic circulation. Drugs with high first-pass metabolism have reduced oral bioavailability — alternative routes (sublingual, transdermal, parenteral) or higher oral doses may be necessary.

Half-life (t½) and steady state
• Half-life is the time required for the drug concentration to fall by 50%. It reflects combined effects of metabolism and elimination. Short half-life (≈4–8 hours) means faster elimination and often more frequent dosing; long half-life (≥24 hours) means prolonged presence and less frequent dosing.
• Steady state is achieved after approximately 3 to 5 half-lives of consistent dosing; at steady state the rate of drug intake equals rate of elimination. Example: digoxin (half-life ≈36 hours in normal renal function) requires several days to reach steady state.

Active metabolites
• Some drugs produce metabolites that are equally or more active than the parent compound (e.g., certain morphine metabolites), altering the duration and nature of effect and sometimes increasing toxicity.

Nursing insights
• Review hepatic function tests (AST, ALT, total bilirubin) when prescribing drugs predominantly metabolized by the liver.
• Anticipate dose reductions or alternative therapies in hepatic impairment.
• Recognize potential enzyme-mediated drug–drug interactions and consult pharmacy when adding or removing medications.
• Consider genetic variability where clinically significant (e.g., codeine activation via CYP2D6).

EXCRETION (ELIMINATION)

Primary routes
• Renal elimination (urine) is the most common route: kidneys filter free, water-soluble drugs and metabolites; active tubular secretion and reabsorption further influence final excretion. Other routes include bile/feces, lungs (volatile agents), sweat, saliva, and breast milk.

Renal function & drug clearance
• Glomerular filtration rate (GFR) and creatinine clearance (CLcr) are key determinants of renal elimination. Renal impairment (acute or chronic) reduces clearance and increases the risk of drug accumulation and toxicity.
• Creatinine clearance is used to adjust dosing for renally excreted drugs; CLcr varies with age, gender, and muscle mass (lower in older adults and women).

Urine pH manipulation
• Urine pH alters tubular reabsorption: acidic urine promotes excretion of weak bases; alkaline urine promotes excretion of weak acids. Clinically, sodium bicarbonate may be used to alkalinize urine to enhance elimination of aspirin (salicylate) in overdose. Conversely, substances that acidify urine (e.g., large amounts of cranberry juice) could theoretically reduce excretion of weak acids.

Extra-renal elimination
• Biliary excretion: drugs excreted in bile may be reabsorbed via enterohepatic circulation, prolonging effect.
• Pulmonary excretion: volatile anesthetics are eliminated via exhalation.

Nursing insights
• Monitor BUN, serum creatinine, CLcr, and urine output to detect changes in elimination capacity.
• Adjust dosing intervals and/or doses for renal impairment; consult dosing guidelines for renally cleared drugs.
• Avoid or use caution with nephrotoxic drugs (aminoglycosides, certain antivirals, NSAIDs) in patients with renal dysfunction.
• Ensure adequate hydration when appropriate to maintain renal perfusion and elimination.

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Pharmacodynamic Phase

(What the drug does to the body — mechanism and effect)

Definition & scope
• Pharmacodynamics concerns the biochemical and physiologic effects of drugs and the mechanisms by which those effects are produced, including receptor interactions, dose–response relationships, efficacy, potency, and the time course of pharmacologic effects.

Primary vs secondary effects
• Primary effect: the therapeutic, intended response (e.g., antihypertensive lowering blood pressure).
• Secondary effect: any additional effect (beneficial or adverse) that occurs because of the drug’s mechanism (e.g., diphenhydramine — primary: antihistamine; secondary: sedation).

Dose–response & maximal efficacy
• Dose–response relationship: increasing dose generally increases effect up to a point. Potency refers to the amount of drug needed to produce a given effect; efficacy refers to the maximal effect achievable. Example: morphine has greater maximal analgesic efficacy than tramadol even if doses are increased.

Onset, peak, and duration
• Onset: time to reach the minimum effective concentration (MEC) after administration.
• Peak: time of highest plasma concentration (and often maximum effect).
• Duration: period during which the drug concentration remains above MEC. Understanding these guides timing of doses and monitoring.

Receptor theory and receptor families
• Drugs typically act by binding to receptors — protein structures on cell membranes or inside cells that mediate effects. The quality of fit between drug and receptor dictates magnitude of response. Major receptor families:

Kinase-linked receptors: ligand binds extracellular domain, activates intracellular kinase → modifies cellular enzymes/transcription. Response typically intermediate in onset.
Ligand-gated ion channels: binding opens ion channel (Na⁺, Ca²⁺) → very rapid responses (milliseconds to seconds).
G protein–coupled receptors (GPCRs): receptor activates a G protein which then alters effectors (enzymes, ion channels) — common and versatile signaling mechanism.
Nuclear receptors: located inside the cell nucleus; activation influences gene transcription and protein synthesis — effects are slower in onset but prolonged.

Agonists, antagonists, partial agonists
• Agonist: binds receptor and elicits full biologic response (e.g., epinephrine on beta receptors).
• Antagonist: binds receptor and blocks agonist action (no intrinsic activity). IC50 quantifies the antagonist concentration required to inhibit 50% of maximum response.
• Partial agonist: elicits less than maximal response even at full receptor occupancy.

Nonselective & nonspecific effects
• Nonspecific: drug acts on physical or chemical processes not limited to one receptor type (e.g., osmotic diuretics).
• Nonselective: drug affects multiple receptor subtypes leading to varied effects at different organs (e.g., bethanechol stimulates cholinergic receptors in bladder, heart, bronchi, and GI tract → bladder contraction + bradycardia + bronchoconstriction + increased gastric secretions).

Categories of drug action

Stimulation or depression of cellular activity (e.g., stimulants vs sedatives).
Replacement therapy (e.g., insulin replaces endogenous hormone).
Inhibition or killing of organisms (antibiotics, antifungals).
Irritation (laxatives that increase peristalsis by local irritation).

Therapeutic index & therapeutic range (window)
• Therapeutic index (TI) = LD50 / ED50. TI is a crude measure of safety: a small TI (close to 1) indicates narrow margin between effective and lethal doses; such drugs require careful monitoring.
• Therapeutic range = plasma concentrations between MEC and minimum toxic concentration (MTC). Some drugs (e.g., digoxin, aminoglycosides, anticonvulsants) have narrow therapeutic ranges and require periodic serum level checks.

Peak & trough monitoring
• Peak level: indicates rate/degree of absorption; drawn at time of expected peak (varies by route: e.g., 1–3 hours post-oral or ~10 min after IV bolus for many drugs).
• Trough level: indicates elimination; drawn immediately before the next dose.
• Monitoring prevents underdosing (low peak) and toxicity (high trough).

Loading dose & digitalization
• A loading dose is an initial large dose given to rapidly achieve therapeutic plasma concentration; subsequent maintenance doses sustain that level. Digoxin digitalization is a classic example where loading doses achieve therapeutic steady state more quickly.

Side effects, adverse reactions, and toxicity
• Side effects: expected, often predictable physiologic effects related to mechanism (may be harmless or bothersome).
• Adverse reactions: more severe, unintended, and undesirable effects occurring at normal doses (e.g., anaphylaxis). Always report and document.
• Toxicity: excessive drug levels produce harmful effects (often preventable with monitoring). Management may require dose reduction, antidotes, or enhanced elimination.

Pharmacogenetics
• Genetic variations alter pharmacokinetics and pharmacodynamics (e.g., metabolism rates differ by CYP genotype), leading to variable drug responses and risk of adverse effects. Ethnic and genetic population differences may influence drug selection and dosing; individual genotyping is increasingly relevant for certain drugs.

Tolerance & tachyphylaxis
• Tolerance: gradual decrease in response over time requiring higher doses for same effect (seen with opioids).
• Tachyphylaxis: rapid development of decreased responsiveness (acute tolerance), seen with some sympathomimetics, nitrates, and psychotropic agents.

Placebo effect & ethical considerations
• Placebo responses are real psychophysiologic improvements due to expectation rather than pharmacologic action; their use is limited by ethical obligations to informed consent in clinical practice.

Nursing insights
• Know whether a drug is an agonist, antagonist, or partial agonist and the clinical implications of each.
• Monitor for expected therapeutic effect and for known class-specific adverse effects.
• Understand drug onset/peak/duration to coordinate assessments, safety precautions (e.g., fall risk when peak sedation expected), and patient education.
• Obtain baseline and periodic laboratory monitoring when indicated (serum drug levels, LFTs, renal function).
• Be aware of genetic and physiologic patient factors that may alter response.

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Additional Concepts And Clinical Applications

Half-life and steady-state application
• Use half-life to anticipate accumulation, duration, and time to steady state. Multiple half-lives are required to eliminate a drug (>90% elimination after about 4–6 half-lives). Adjust dosing frequency based on half-life to avoid accumulation.

Drug interactions — protein binding and metabolism
• Protein displacement: two highly protein-bound drugs coadministered may increase free concentrations of one or both agents → toxicity.
• Metabolic interactions: enzyme inhibitors (e.g., some antifungals) and inducers (some anticonvulsants) change plasma drug levels — monitor closely and adjust dosing.

Special populations
• Pediatrics: immature hepatic enzymes, higher body water proportion, and higher gastric pH alter pharmacokinetics; dosing often weight-based and requires age-specific considerations.
• Older adults: decreased renal function, reduced hepatic blood flow, altered body composition (higher fat:lean ratio, lower albumin) → altered distribution, metabolism, and excretion; start low and go slow when dosing.
• Pregnancy & lactation: evaluate teratogenic risk, placental transfer, and excretion into breast milk; weigh maternal benefit vs fetal/neonatal risk.

Routes that bypass the pharmaceutic phase
• Parenteral (IV, IM, subQ), transdermal, sublingual, inhalational, ophthalmic, and intranasal routes bypass or significantly alter the pharmaceutic phase and have distinct absorption profiles and monitoring needs.

Practical nursing actions for safe medication use
• Always verify route and formulation before administration.
• Know which medications must not be crushed.
• Check compatibility for IV mixtures and infusion rates.
• Confirm allergy history and document adverse reactions.
• Monitor therapeutic and adverse effects and appropriate labs (drug levels, renal and hepatic function).
• Educate patients on timing relative to meals, expected onset, and warning signs of toxicity or adverse effects.
• Report and document adverse drug events per facility policy.

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Summary

Three phases of drug action:

Pharmaceutic (oral solids only — disintegration/dissolution)
Pharmacokinetic (ADME)
Pharmacodynamic (drug effect and mechanism).

• Pharmaceutic phase: affected by formulation (enteric-coated, sustained-release), excipients, gastric pH, motility, and presence of food. Do not crush enteric-coated or extended-release forms.
• Absorption: route, blood flow, lipid solubility, ionization, surface area, and first-pass hepatic metabolism influence how much and how fast a drug reaches systemic circulation. Bioavailability varies by route and formulation.
• Distribution: blood flow, tissue affinity, protein binding, and barriers (BBB, placenta) determine drug movement to site of action. Hypoalbuminemia increases free drug and toxicity risk. Volume of distribution influences half-life.
• Metabolism: primarily hepatic via enzyme systems; can inactivate drugs or produce active metabolites. Genetic differences and liver disease greatly affect metabolism. Half-life and steady state are functions of metabolism/elimination.
• Excretion: kidneys are the primary route for most drugs/metabolites; renal impairment necessitates dose adjustment. Urine pH can be manipulated to enhance elimination of certain drugs.
• Pharmacodynamics: drugs act via receptors, enzymes, ion channels, and other mechanisms; potency and efficacy differ between agents. Therapeutic index and therapeutic range determine monitoring needs.
• Monitoring & safety: peak/trough levels for narrow therapeutic index drugs, LFTs/renal tests for drugs cleared by those organs, vigilance for adverse reactions and interactions.
• Clinical implications: age, pregnancy, organ dysfunction, genetic variability, and polypharmacy require individualized dosing, close monitoring, and patient education.

Nursing Insights:
• Verify formulation before administration (do not crush SR/ER/enteric-coated).
• Confirm route appropriateness for clinical status (IV when rapid effect required or absorption impaired).
• Assess hepatic and renal function and adjust dose as indicated; obtain CLcr when dosing renally excreted drugs.
• Monitor serum drug levels where applicable (aminoglycosides, anticonvulsants, digoxin).
• Evaluate for drug interactions (protein binding displacement, enzyme induction/inhibition).
• Educate patients about administration timing with food, potential side effects, and signs of toxicity.
• Consult pharmacy for complex dosing decisions, interactions, and alternative formulations.

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