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).

First-Order Elimination Kinetics Most drugs follow first-order kinetics — 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: drugs are typically re-dosed every 1 half-life to maintain therapeutic levels. 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: 3–9 hours (effect lasts longer than half-life due to irreversible platelet binding). Diazepam (Valium): 20–100 hours (active metabolites extend effect to days). Fluoxetine (Prozac): 1–4 days (fully cleared in 4–16 days — explains the slow start/stop effect). 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): acute phase 1–2 hours; fat-stored metabolites 3–30 days depending on frequency of use.

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 — the drug level plateaus. This is why antidepressants take 2–4 weeks to reach full therapeutic effect.

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.