Pharmacokinetics

Pharmacokinetics is the branch of pharmacology that describes what the body does to a drug — in contrast to pharmacodynamics, which describes what the drug does to the body. Pharmacokinetics follows a drug through four phases, abbreviated ADME: absorption, distribution, metabolism, and excretion.

Absorption is the drug's movement from its administration site into the bloodstream; the fraction that arrives intact is its bioavailability. An intravenous dose is 100% bioavailable, while an oral dose may lose much of its strength to incomplete absorption and the first-pass effect — metabolism by the liver before the drug ever reaches systemic circulation. Distribution then carries the drug to tissues, shaped by blood flow, protein binding, and barriers like the blood-brain barrier. Metabolism, mostly hepatic and driven by cytochrome P450 enzymes, converts drugs into metabolites that are usually less active and easier to eliminate. Excretion removes drug and metabolites, primarily through the kidneys into urine.

These processes set the numbers clinicians dose by: half-life (the time for the drug's concentration to fall by half), clearance, volume of distribution, and steady-state timing. They also explain many drug-drug interactions — one drug inducing or inhibiting the P450 enzymes that metabolize another can push levels dangerously low or high, a central concern with narrow therapeutic index medications.

Pharmacokinetics is foundational on multiple exams. The USMLE Step 1 tests ADME concepts and calculations like clearance and loading doses, the CCMA covers pharmacokinetic principles within pharmacology basics, and the PTCE emphasizes pharmacokinetic drug-drug interactions in pharmacy technician practice.