Half-life

Half-life is the time it takes for the concentration of a substance in the blood to fall by half after absorption is complete.

The figure summarizes how quickly the body eliminates a compound and is calculated from the declining part of the concentration-time curve. It determines how long effects may last, how often a dose must be repeated to maintain a stable level, and how long a substance may remain detectable. As a rule of thumb, most of a compound is gone after four to five half-lives. Half-life depends on both clearance and the volume of distribution.

Cannabidiol is fat-soluble and accumulates in adipose tissue, so its half-life is longer after repeated use than after a single dose, and reported values vary considerably across studies, routes, and formulations. This storage effect is one reason cannabinoids can be detected for some time after use stops. Readers meet the term in pharmacokinetic study summaries and drug-testing discussions. Published half-lives are population averages and do not predict how long any individual retains the compound.