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Pharmacokinetics & Pharmacodynamics Essentials Chapter: Pharmacokinetics and Pharmacodynamics
🖨 Print or save as PDF Updated 2026-10-20
Pharmacokinetic terms Term Meaning Quick use Bioavailability (F) Fraction of a dose reaching the systemic circulation unchanged IV = 100%; oral is lower because of absorption and first-pass liver metabolism Volume of distribution (Vd) Amount in body ÷ plasma concentration Loading dose = target concentration × Vd Clearance (CL) Volume of plasma cleared of drug per unit time Maintenance dose rate = CL × target concentration Half-life (t½) Time for the plasma concentration to fall by half (t½ = 0.693 × Vd ÷ CL) Steady state and near-complete elimination take about 4–5 half-lives (94–97%) First-order kinetics A constant FRACTION eliminated per unit time Most drugs Zero-order kinetics A constant AMOUNT eliminated per unit time Ethanol, and phenytoin and aspirin at high doses
Example: 100 mg with a 4-hour half-life → after 12 hours (3 half-lives) 12.5 mg remain. Metabolism and interactions Topic Key points Phase I Oxidation, reduction, hydrolysis (mostly cytochrome P450); can create active or toxic metabolites Phase II Conjugation (glucuronidation, sulfation, acetylation) → more water-soluble for renal excretion CYP3A4 inhibitors (raise levels of other drugs) Erythromycin, clarithromycin (not azithromycin), azole antifungals, grapefruit juice, some HIV protease inhibitors CYP inducers (lower levels) Rifampin, carbamazepine, phenytoin, St. John's wort, chronic alcohol Prodrugs Codeine and tramadol (activated by CYP2D6), clopidogrel (CYP2C19), valacyclovir (to acyclovir)
Pharmacodynamic terms Term Meaning Potency Dose needed for a given effect (EC₅₀); curve farther left = more potent Efficacy Maximum effect (Emax) a drug can produce, regardless of dose Full vs partial agonist Partial agonist cannot reach the full Emax and can block a full agonist (e.g., buprenorphine at μ receptors) Competitive antagonist Shifts the curve right; Emax unchanged (overcome by more agonist), e.g., naloxone Noncompetitive antagonist Lowers Emax; cannot be overcome by more agonist Therapeutic index TD₅₀ ÷ ED₅₀ (or LD₅₀ ÷ ED₅₀); small TI (warfarin, lithium, digoxin, phenytoin) means close monitoring
⚠️ Common traps Confusing potency with efficacy: a drug can be more potent but less effective. Saying azithromycin is a strong CYP3A4 inhibitor like erythromycin and clarithromycin. Applying a fixed fraction rule to zero-order drugs such as ethanol. Thinking a competitive antagonist lowers the maximum effect. Mini-quiz Pick an answer to see right away whether it is correct and why.
A drug has a half-life of 6 hours. About how long until it is essentially eliminated after stopping?
A. 24–30 hours B. 6 hours C. 12 hours D. 72 hours
Drug A reaches its maximum effect at 10 mg; drug B reaches a higher maximum at 50 mg. Which statement is correct?
A. Drug B is more potent and more efficacious B. Drug A is more potent; drug B is more efficacious C. Drug A is more efficacious D. They have the same efficacy
Which antibiotic is most likely to raise blood levels of a statin metabolized by CYP3A4?
A. Amoxicillin B. Azithromycin C. Penicillin VK D. Clarithromycin
A drug with TD₅₀ = 400 mg and ED₅₀ = 20 mg has a therapeutic index of:
A. 0.05 B. 380 C. 20 D. 8,000
Which drug follows zero-order elimination at usual doses?
A. Amoxicillin B. Lidocaine C. Ibuprofen D. Ethanol
0 of 5 answered
Answer key: 1 A · 2 B · 3 D · 4 C · 5 D
