Pharmacokinetics and Biopharmaceutics
Core concepts
Understand absorption, distribution, metabolism, and excretion quantitatively. Know first-order vs zero-order kinetics, half-life, volume of distribution, clearance, and bioavailability (F). Be able to use the relationships: t½ ≈ 0.693 × Vd/CL; loading dose = (Vd × target concentration)/F; maintenance dose = (CL × target concentration × dosing interval)/F. Understand steady state (about 4–5 half-lives), accumulation, and the difference between peak and trough monitoring. Know the first-pass effect, enterohepatic recycling, protein binding, and how renal or hepatic impairment changes clearance. In biopharmaceutics, understand dissolution vs permeability limits (BCS) and bioequivalence criteria.
Question traps
The most common error is mixing up Vd and clearance - Vd determines loading dose, clearance determines maintenance dose. Another trap is assuming half-life changes when only Vd changes, or forgetting to adjust for bioavailability. Questions about zero-order drugs (e.g., phenytoin, ethanol, high-dose aspirin) often test that a constant amount, not a constant fraction, is eliminated, so small dose increases can cause large level jumps.
Worked rule of thumb
If a drug has a half-life of 8 hours, steady state is reached in roughly 32–40 hours (4–5 half-lives). For a patient needing a rapid therapeutic level, give a loading dose based on Vd; use clearance-based maintenance dosing thereafter.
Study approach
Memorize the four core equations and practice one calculation per concept until the units are automatic.