Half-life (t½) is the time required to reduce the concentration of a drug by half. The formula for half-life is (t½ = 0.693 × Vd /CL) Volume of distribution (Vd) and clearance (CL) are required to calculate this variable. 0.693 is the logarithm of 2, and represents the exponential rate of elimination (assuming elimination is by first order kinetics) Half life is increased by an increase in the volume of distribution and inc...
Half-life (t½) is the time required to reduce the concentration of a drug by half. The formula for half-life is (t½ = 0.693 × Vd /CL) Volume of distribution (Vd) and clearance (CL) are required to calculate this variable. 0.693 is the logarithm of 2, and represents the exponential rate of elimination (assuming elimination is by first order kinetics) Half life is increased by an increase in the volume of distribution and increased by a decrease in the rate of clearance. In some disease states (eg. renal failure with oedema) volume of distribution increases but clearance decreases, resulting in an unchanged half life (thus, it is a poor measure of drug clearance alone). It takes about 5 half-lives for a drug to be roughly 97% eliminated. (50%, then 75% then 87.5% then 93.75% then 96.875%). Doubling the dose of a drug will usually increase its duration of action by one half-life (because its clearance is a logarithmic function) For drugs eliminated by first-order kinetics, half life is constant regardless of concentration. With zero-order kinetics the term becomes meaningless, and one refers instead to a dose or concentration removed over time. https://derangedphysiology.com/main/cicm-primary-exam/required-reading/pharmacokinetics/Chapter%20322/half-life
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