The biological half-life or elimination half-life of a substance is the time it takes for a substance (for example a metabolite, drug, signalling molecule, radioactive nuclide, or other substance) to lose half of its pharmacologic, physiologic, or radiologic activity, as per the MeSH definition. In a medical context, half-life may also describe the time it takes for the blood plasma concentration of a substance to halve ("plasma half-life") its steady-state. The relationship between the biological and plasma half-lives of a substance can be complex depending on the substance in question, due to factors including accumulation in tissues, active metabolites, and receptor interactions.
Biological half-life is an important pharmacokinetic parameter and is usually denoted by the abbreviation t½.
While a radioactive isotope decays perfectly according to first order kinetics where the rate constant is fixed, the elimination of a substance from a living organism, into the environment, follows more complex kinetics. See the article rate equation.
Read more about Biological Half-life: Sample Values and Equations
Famous quotes containing the words biological, half-life:
“Although there is no universal agreement as to a definition of life, its biological manifestations are generally considered to be organization, metabolism, growth, irritability, adaptation, and reproduction.”
—The Columbia Encyclopedia, Fifth Edition, the first sentence of the article on “life” (based on wording in the First Edition, 1935)
“I could draw Bloom County with my nose and pay my cleaning lady to write it, and I’d bet I wouldn’t lose 10% of my papers over the next twenty years. Such is the nature of comic-strips. Once established, their half-life is usually more than nuclear waste.”
—Berkeley Breathed (b. 1957)