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Half-Life

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