Drug Half-Life Calculator

A drug half-life is the time required for the amount or concentration of a medicine in the body to fall by one half during an elimination phase. Under a simple first-order model, the same fraction disappears during every equal time interval. After one half-life, 50 percent remains; after two, 25 percent remains; after three, 12.5 percent remains. This calculator rearranges that exponential relationship to solve for any one missing value. The model uses remaining amount = initial amount multiplied by 0.5 raised to elapsed time divided by half-life. If 500 mg is treated as the initial amount and the half-life is six hours, 250 mg remains at six hours and 125 mg remains at twelve hours. To solve for elapsed time or half-life, the calculator uses logarithms. Values must use matching units: both amounts might be mg, g, micrograms, IU, or concentration units, while both time values might be minutes, hours, or days. Half-life does not usually mean that a drug is completely gone after that interval. Exponential decay approaches zero gradually. A common approximation is that about 97 percent has been eliminated after five half-lives, although measurable traces can remain longer. Repeated dosing also changes the picture because each new dose is added to medication still present from earlier doses. With regular dosing, many drugs approach a steady state after roughly four to five half-lives. Real pharmacokinetics can be more complex than a single exponential curve. Absorption, distribution into tissues, active metabolites, multiple elimination phases, kidney or liver impairment, age, body composition, interactions, genetic differences, and dose-dependent metabolism can all alter observed concentrations. A published population half-life is an estimate, not a precise timer for one person. Extended-release products and medicines with active metabolites may require a more detailed model. Use this result for education and arithmetic checks only. Never use it to start, stop, repeat, or change a medication dose without instructions from a qualified prescriber or pharmacist. Toxicity, withdrawal, therapeutic drug monitoring, and overdose require professional assessment rather than a generic half-life estimate. If a dose was missed or taken incorrectly, follow the medicine label and contact an appropriate clinician, pharmacist, poison service, or emergency service based on the circumstances.