What is pharmacokinetics?

The core idea

Pharmacokinetics is the study of what the body does to a drug: how it moves into, through and out of the body over time. It is usually taught as four processes - absorption, distribution, metabolism and excretion (ADME). Together they determine how much of a dose reaches its target, how fast, and for how long.

Its partner concept is pharmacodynamics, which covers what the drug does to the body: the effect produced at a given concentration.

Absorption and distribution

Absorption is how a drug enters the bloodstream from its route of administration. A swallowable tablet crosses the gut wall and passes the liver first (first-pass metabolism), which can sharply reduce how much active drug arrives; an intravenous dose skips absorption entirely. Bioavailability names the fraction of a dose that reaches the circulation in active form.

Distribution is where the drug goes next: some drugs stay mostly in blood, others spread widely into fat or tissue. Protein binding, blood flow and barriers such as the blood-brain barrier shape this.

Metabolism and excretion

Metabolism, mostly in the liver, chemically changes the drug - usually toward inactive, more water-soluble products, though some drugs are activated by metabolism (prodrugs). Excretion removes the drug and its metabolites, mainly through the kidneys into urine.

Two measurable quantities summarize the whole chain: half-life, the time for the blood concentration to fall by half; and clearance, the volume of blood cleared of drug per unit time. Dosing intervals and dose adjustments in kidney or liver disease are built from these.

Why it matters

Pharmacokinetics explains everyday prescribing facts: why some drugs are taken with food, why grapefruit juice can dangerously raise levels of drugs metabolized by the CYP3A4 enzyme, why one drug is dosed once daily and another four times a day, and why the same dose can be too much for one patient and too little for another.