The amount of time it takes for a drug's concentration in the body to decrease by 50% is called the drug's half-life (t1/2).

The longer a drug's half-life, the slower it is removed from the body. Most drugs are eliminated from the body in 1 to 3 days, but some drugs with longer half-lives can still be detected in the body weeks after the initial dose.

The figure below illustrates a typical kinetic pattern for an oral drug.