Pharmacokinetic Terms Explained

Pharmacokinetics describes what happens to a compound over time after it enters a system. Plot measured concentration against time and the result is a curve that rises, peaks and falls. Nearly every term below is a coordinate on that curve or a property of its shape, which is why they are easier to hold together than to memorise separately.

• The rise reflects absorption into the sampled compartment
• The peak is the highest concentration reached
• The fall reflects distribution, metabolism and elimination
• Every value is measured in a stated matrix, usually plasma

Cmax is the highest concentration observed; Tmax is the time at which it occurred. They are always reported as a pair because neither is interpretable alone — the same Cmax reached in twenty minutes and in eight hours describes two very different behaviours.

• Cmax is a concentration, in units such as ng/mL
• Tmax is a time, in hours or minutes from administration
• Both are observed values: they depend on when samples were actually taken
• Sparse sampling can miss a true peak and report a later, lower one

Half-life is the time taken for concentration to fall by half during the elimination phase. It is a derived value rather than a measured one, and a study may report several — a distribution half-life for the early steep fall and a terminal half-life for the slower tail — which is why two papers on the same compound can quote very different numbers without either being wrong.

• Derived from the slope of the decline, not read off a single sample
• Distribution and terminal half-lives describe different phases
• A longer sampling window can reveal a longer terminal half-life
• Reported as t½, sometimes with a subscript naming the phase

The area under the concentration-time curve summarises total exposure over a period rather than at a moment. It is how two compounds, or two conditions, are compared when their curves have different shapes — one may peak higher while the other sustains longer, and AUC is what makes that comparison a single number.

• Expressed over a stated interval, such as AUC 0-24h
• Two very different curves can share an AUC
• Reported alongside Cmax rather than instead of it
• Sensitive to how long sampling continued

A pharmacokinetic figure is a measurement made in a specific system: a species, a model, a route, a dose and a sampling schedule. Move any of those and the number moves with it. A half-life measured in a rodent model is not a half-life in a human, and an in-vitro result is not a pharmacokinetic result at all. Every value published on this site is shown with the population the source describes, and where no identifiable source states a value, the page says so instead of showing a figure.

• Species and model change the value, often by a large factor
• Route and dose change absorption and therefore Tmax and Cmax
• Sampling schedule bounds what the study could have observed
• A figure quoted without its population cannot be compared with another