Drug Half-Life in Body Calculator

What Is a Drug’s Half-Life?

The biological half-life (t½) is the time it takes for the concentration of a drug in the body to fall to half its current value.
This is different from the nuclear half-life. It refers specifically to how fast the body eliminates a medication.
After 1 half-life: 50% remains. After 2: 25%. After 3: 12.5%. After 4: 6.25%. After 5: ~3.1%.
Most drugs are considered fully eliminated after 4–5 half-lives (~97% cleared).

This page is about one dose. You took something, and you want to know how much is left and when it is gone. The calculator gives the exact time to 95% and 97% clearance rather than rounding to whole half-lives, so its figures sit slightly beyond the “5 half-lives” shorthand.
If you are taking a drug repeatedly and want the steady-state level it builds to, that is the drug accumulation and steady-state calculator. Same first-order model, different question, and it takes the dosing interval as an input.

First-Order Elimination Kinetics

Most drugs follow first-order kinetics, meaning a constant fraction is eliminated per unit time.
C(t) = C₀ × e^(−0.693 × t / t½)
Where C₀ is the initial concentration, t is time elapsed, and t½ is the half-life.
This means the absolute amount eliminated decreases over time, but the fraction eliminated stays constant.

Why Half-Life Matters

Dosing intervals: the half-life sets how often a drug has to be taken to stay in range, though rarely one for one. Amoxicillin’s half-life is about an hour and it is given every 8 or 12 hours, because the dose is large enough to stay above what the bacteria need for most of that gap.
Washout period: doctors need to know when a drug is fully cleared before switching medications.
Drug interactions: some drugs inhibit or induce enzymes (CYP450) that metabolize other drugs, effectively changing their half-life.
Detection windows: drug tests detect metabolites for several half-lives after last dose.

Common Drug Half-Lives (approximate)

Ibuprofen: 2 hours (fully cleared in ~10 hours).
Aspirin: about 20 minutes for aspirin itself, which the body turns into salicylate. Salicylate lasts 2 to 3 hours at low doses and much longer at high ones, and the effect on platelets outlasts both, because it is permanent for the life of each platelet (7 to 10 days).
Diazepam (Valium): 20–100 hours (active metabolites extend effect to days).
Fluoxetine (Prozac): 1–4 days for the drug itself, but its active metabolite norfluoxetine runs 4 to 16 days, so the effect takes weeks to fade. The label asks for a 5-week gap before starting an MAOI (monoamine oxidase inhibitor).
Amoxicillin: 1 hour (dosing every 8 hours to maintain therapeutic levels).
Metformin: 6.2 hours (mostly cleared in 31 hours).
Warfarin: 36–42 hours (requires careful dosing adjustment).
THC (cannabis): blood levels fall within hours, but THC is stored in fat, so its terminal half-life is about 1 to 2 days in an occasional user and 5 to 13 days in a heavy one. Urine tests pick up its metabolite for days to weeks for the same reason.

Accumulation with Repeated Dosing

When a drug is taken repeatedly, it accumulates until reaching steady state.
Steady state is reached after approximately 4–5 half-lives.
At steady state, the amount eliminated per dose equals the amount taken, so the drug level plateaus.
Steady state is not why antidepressants take weeks to work, though it is often given as the reason. Sertraline plateaus in about a week, and the 2 to 6 week delay is the brain adapting rather than the level building up. Fluoxetine is the one where both run long: with norfluoxetine it takes a month or more to plateau.

Renal and Hepatic Considerations

Drugs cleared by the kidneys have longer half-lives in patients with kidney disease (low eGFR).
Drugs metabolized by the liver have longer half-lives in liver disease or with enzyme-inhibiting drugs.
Always consult a pharmacist or physician when adjusting doses based on organ function.

Disclaimer

This tool is for educational purposes only. Do not make medication decisions without consulting a licensed healthcare provider.

An important exception: zero-order kinetics. Most drugs follow first-order kinetics (the half-life model above), where a constant fraction clears per hour. But a few important ones do not. Alcohol is the canonical example: above a low threshold, the liver’s enzymes are saturated and the body eliminates a roughly constant amount per hour (about one drink’s worth) regardless of how much is present. Phenytoin (an anti-seizure drug) does the same thing at therapeutic concentrations. For these zero-order drugs, the half-life model breaks down: clearance time depends linearly on starting amount, not on logarithmic doublings. Always check whether a drug follows first- or zero-order kinetics before applying half-life math.