pharmacokinetics

Pharmacokinetics is the study of what the body does to a drug over time. If pharmacodynamics is what the drug does at its destination, pharmacokinetics is the whole journey: getting in, traveling around, being broken down, and being shown out. It is the logistics of the drug rather than its action.

The journey is usually summarized by four steps, abbreviated ADME: absorption (how the drug gets into the bloodstream), distribution (how it spreads to tissues), metabolism (how the body chemically alters it, mostly in the liver), and excretion (how it leaves, mostly via the kidneys). These steps together determine the concentration of drug at the site of action moment by moment.

Pharmacokinetics turns these processes into useful numbers: bioavailability, clearance, half-life, and volume of distribution, among others. These let clinicians predict how high a drug's concentration will climb, how long it will last, and how often to dose. Crucially, the body's handling of a drug varies with age, kidney and liver function, genetics, and other drugs, so the same dose can produce very different concentrations in different people.