Biological half-life

Biological half-life (elimination half-life, pharmacological half-life) is the time taken for the concentration of a biological substance, such as a medication, to decrease from its maximum initial concentration (Cmax) to the half of Cmax in the blood plasma. It is denoted by the abbreviation

t

1

2

{\displaystyle t_{\frac {1}{2}}}

.

In multi-compartment pharmacokinetics, two operational half-lives are often distinguished: an early distribution (α) half-life governed by redistribution from the central to peripheral compartments, and a later elimination (β) half-life governed by metabolic clearance and excretion.

This is used to measure the removal of things such as metabolites, drugs, and signalling molecules from the body. Typically, the biological half-life refers to the body's natural cleansing, the detoxification through liver metabolism and through the excretion of the measured substance through the kidneys and intestines. This concept is used when the rate of removal is roughly exponential.

In a medical context, half-life explicitly describes the time it takes for the blood plasma concentration of a substance to halve (plasma half-life) its steady-state when circulating in the full blood of an organism. This measurement is useful in medicine, pharmacology and pharmacokinetics because it helps determine how much of a drug needs to be taken and how frequently it needs to be taken if a certain average amount is needed constantly. By contrast, the stability of a substance in plasma is described as plasma stability. This is essential to ensure accurate analysis of drugs in plasma and for drug discovery.