The half-life is the time in which half of a psychoactive substance is metabolized or excreted in the body. The half-life can therefore be used to describe how long the body needs until a psychoactive substance is metabolized or excreted. The half-life is therefore central to determining the dosing intervals.

An example: After the duration of a half-life, the substance is not completely degraded, but only half of it. This means that the remaining residue (half) takes just as long to be half degraded or excreted - so after two half-lives a quarter of the original amount of substance remains in the body, after three half-lives an eighth.