Half-Life
Definition
The time for a measured amount or concentration to fall by half under specified conditions. In drug studies, the reported half-life depends on the elimination phase and model.
By PurePeptideHub Editorial Team · Updated . Educational source review; independent clinical review pending.
A numerical example
For a hypothetical first-order process with a two-hour half-life, an amount falls from 100 to 50 units after two hours, then to 25 after four hours and 12.5 after six. The same fraction disappears each interval. This example assumes no additional input and is not a dosing schedule.
References: [1]
Which half-life is being reported?
A paper may report a distribution phase, a terminal elimination phase or a biological response that lasts longer than detectable drug exposure. Record the measured substance, sample type and phase before comparing values. A peptide fragment or metabolite can have different behavior from the parent molecule.
Half-life versus shelf life
Shelf life concerns product stability during storage. Plasma half-life concerns behavior after exposure in a biological system. Neither one alone tells you how often a medicine should be used. Formulation, efficacy and safety data are also needed.