Article — Drug Half-Life Calculator
Drug half-life calculator
Drug half-life (t½) is the time required for the plasma concentration of a drug to fall by half. Concentration follows first-order decay: C(t) = C₀ × (½)^(t/t½). After 5 half-lives, about 97% has been eliminated. Ibuprofen has a 2-hour half-life. Caffeine, 5–6 hours. Warfarin, around 40 hours. Amiodarone, up to 142 days.
This calculator uses simplified first-order pharmacokinetics for educational purposes. It does not account for absorption phase, distribution to tissues, active metabolites, protein binding, drug interactions, renal or hepatic dysfunction, or genetic variation in metabolism. Do not adjust any medication dose based on its output. Consult a licensed healthcare professional for any dosing decision.
What is drug half-life?
The half-life of a drug is the time it takes for half of the substance in the bloodstream to be eliminated by the body — through liver metabolism, kidney excretion, exhalation or other pathways combined. The concept assumes first-order kinetics, which is true for most drugs at therapeutic concentrations. The body removes a constant fraction per unit time, not a constant amount.
First-order kinetics produce an exponential decay curve. After one half-life, 50% remains. After two half-lives, 25%. After three, 12.5%. The pattern continues: 6.25%, 3.13%, 1.56%, and so on. The asymptote is zero but is never literally reached.
Amiodarone, an antiarrhythmic, holds the record for longest pharmacological half-life among common prescription drugs: 25 to 142 days depending on tissue. A patient who stops taking it can still have detectable drug levels nine months later. This extreme half-life is why amiodarone is reserved for severe arrhythmias — its effects (and side effects) persist long after the prescription ends.
The drug half-life formula
Two equivalent expressions describe first-order elimination. The half-life form is intuitive; the exponential form is what mathematicians and pharmacokinetic models prefer.
C(t) = C₀ × (½)^(t/t½) half-life formC(t) = C₀ × e^(−kt) exponential formk = ln(2) / t½ ≈ 0.693 / t½ elimination constantt_steady ≈ 5 × t½ steady-state timeAUC = Dose / Cl area under curveThe elimination rate constant k captures how fast the body removes drug. A k of 0.115/h corresponds to a 6-hour half-life. Clearance Cl, often expressed in litres per hour, is volume of distribution Vd times k. These three quantities form the foundation of clinical pharmacokinetics.
Drug half-life clearance rules
The "5 half-lives" rule of thumb defines functional clearance in clinical practice. After 5 half-lives, 96.875% has been eliminated — close enough to call the drug gone for routine purposes. After 10 half-lives, 99.9% has cleared.
- 1 half-life: 50% remains. The drug is at peak when patients often feel best.
- 2 half-lives: 25% remains. Effect is fading; some drugs need redosing here.
- 3 half-lives: 12.5% remains. Often the bottom of the therapeutic window.
- 4 half-lives: 6.25% remains. Most therapeutic effect is gone.
- 5 half-lives: 3.125% remains. Functional clearance for most clinical purposes.
- 7 half-lives: 0.78% remains. Used for drug-washout protocols before clinical trials.
- 10 half-lives: 0.098% remains. Used for prolonged washout, e.g., MAOI before another antidepressant.
Drug half-life vs duration of action
Half-life describes pharmacokinetics — how long the drug is in the body. Duration of action describes pharmacodynamics — how long the clinical effect lasts. The two are correlated but not identical. Some drugs bind tightly to their receptors and produce effects long after plasma levels fall to nothing.
Aspirin is the classic mismatch. Its parent compound has a half-life of 15–30 minutes, but its antiplatelet effect lasts the full 7–10 day lifespan of the platelet because aspirin irreversibly acetylates COX-1. Plasma levels are irrelevant after an hour; the effect persists for days.
Half-lives of common drugs
- Aspirin (acetylsalicylic acid): 15–30 min (parent), 2–3 h (salicylate metabolite)
- Ibuprofen: 1.8–2 h. Dosed every 4–6 h.
- Paracetamol / acetaminophen: 2–3 h. Dosed every 4–6 h.
- Caffeine: 5–6 h in adults; up to 80 h in newborns and pregnant women
- Diphenhydramine (Benadryl): 8–10 h. Why next-day grogginess happens.
- Phenytoin: 6–24 h (saturable kinetics — varies with dose)
- Warfarin: 36–42 h. Daily dosing reaches steady state in about a week.
- Digoxin: 36–48 h (longer in renal impairment)
- Fluoxetine (Prozac): 4–6 days for parent, 4–16 days for active metabolite
- Diazepam (Valium): 20–100 h including active metabolites
- Amiodarone: 25–142 days. Effectively a lifelong drug.
What changes drug half-life
Published half-lives are population averages. Individual patients can metabolize a drug 2–5× faster or slower than the textbook number. Eight major factors shift the actual half-life away from the published value.
- Age: Newborns and elderly metabolize most drugs slower. Caffeine half-life is 80 h in newborns vs 6 h in adults.
- Renal function: Drugs cleared by kidneys accumulate when GFR drops. Digoxin dose must be reduced in renal failure.
- Hepatic function: Cirrhosis slows metabolism of CYP-substrate drugs.
- Genetics: CYP2D6 poor metabolizers (about 7% of Europeans) have 2–4× longer half-life for codeine, tamoxifen, many antidepressants.
- Drug interactions: CYP inhibitors (azoles, macrolides, grapefruit juice) extend half-life of co-administered drugs.
- Pregnancy: Increased volume of distribution and altered enzyme activity change half-lives in either direction.
- Body composition: Highly lipophilic drugs (diazepam, amiodarone) have longer half-life in obese patients.
- Disease state: Heart failure reduces hepatic blood flow and slows clearance of high-extraction drugs.
Common drug half-life mistakes
If a drug has active metabolites, the parent half-life can be misleading. Fluoxetine has a 4-day half-life for the parent compound but norfluoxetine, its active metabolite, persists for up to 16 days. Switching to another antidepressant requires a 5-week washout to avoid interaction.
The most common conceptual error is treating half-life as a precise individual property. It is a statistical average. Two patients with identical doses can have very different half-lives because of genetic, dietary or disease-state differences. Therapeutic drug monitoring (warfarin INR, lithium levels, vancomycin troughs) exists precisely because population half-lives are not reliable for individual dose adjustment.
A second trap is ignoring the absorption phase. First-order elimination kinetics apply once the drug is fully absorbed and distributed. For oral doses, plasma concentration is still rising 1–4 hours after ingestion. The half-life model does not describe this phase — it describes the descending portion of the curve.
A third trap is conflating drug elimination with effect washout. Aspirin is gone from plasma in two hours, but its antiplatelet effect persists for a week. Conversely, a drug like nitroglycerin can have a clinical effect that ends before plasma half-life suggests.