Medication Half-Life Calculator

A drug's half-life (t1/2) is the time it takes for the concentration of a drug in the body to decrease by 50%. Most drugs follow first-order kinetics, meaning a constant fraction (not a constant amount) is eliminated per unit time. After 1 half-life, 50% remains. After 2 half-lives, 25% remains. After 5 half-lives, approximately 97% is eliminated - this is the standard clinical definition of a drug being 'cleared' from the body.

Due to first-order kinetics, each half-life reduces the remaining drug by 50%: 100% > 50% > 25% > 12.5% > 6.25% > 3.125%. After 5 half-lives, only about 3% of the original dose remains - considered clinically negligible for most purposes. After 4 half-lives (93.75% eliminated), most pharmacological effects are minimal. The exact number of half-lives for practical clearance depends on the drug's therapeutic index and the specific clinical situation.

Steady state (when the rate of drug administered equals the rate of elimination) is reached after approximately 4-5 half-lives of regular dosing. This is why drugs with long half-lives (like fluoxetine with a 1-4 day half-life) take weeks to reach therapeutic steady-state levels, while drugs with short half-lives reach steady state within hours. Loading doses can be used to accelerate the time to therapeutic concentrations.

Drug half-life can vary significantly based on: kidney function (renally cleared drugs have longer half-lives in renal impairment); liver function (hepatically metabolised drugs are affected by liver disease); age (older adults often clear drugs more slowly); drug interactions (CYP450 enzyme inhibitors or inducers); genetics (pharmacogenomics affects metabolism rates); and body composition (lipophilic drugs distribute differently in obese patients). Published half-lives are population averages.

No. Published half-lives are averages from clinical pharmacokinetic studies in healthy adults. Actual half-lives can vary 2-10 fold between individuals. Some drugs (like alcohol and phenytoin) follow zero-order kinetics at high concentrations, where a constant amount rather than a fraction is eliminated per time unit - these behave differently from the exponential decay shown in this calculator. Always consult a pharmacist or prescriber about specific drug timing questions.