Drug half-life calculator

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Calculate how long a medication remains in the body based on its half-life. Look up the half-life of 80+ common drugs, or enter a custom half-life. Results show remaining concentration at each half-life interval and time to clinical elimination.

If entered, results will show actual clock times for each half-life.

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Drug half-life calculation
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Drug half-life: what it means and why it matters

Half-life (t½) is one of the most important pharmacokinetic parameters for any drug. It determines how often you need to take it, how long it takes to leave your system, and how long it takes to reach a stable therapeutic concentration when you first start taking it.

The mathematics of half-life

After each half-life, 50% of the remaining drug concentration is eliminated. The formula is: C(t) = C₀ × (0.5)^(t/t½), where C₀ is the initial concentration and t is the elapsed time. After 1 half-life: 50% remains. After 2: 25%. After 3: 12.5%. After 4: 6.25%. After 5: 3.13%. Clinically, a drug is considered essentially eliminated after 4-5 half-lives, when 94-97% has left the system, a figure confirmed by the NIH's clinical reference on drug elimination half-life.

Long half-lives: fluoxetine and diazepam

Fluoxetine (Prozac) has a half-life of 1-6 days for the parent drug, and its active metabolite norfluoxetine has a half-life of 4-16 days. Full elimination can take 4-6 weeks. This is why Prozac's FDA-approved prescribing information contraindicates starting an MAOI within 5 weeks of stopping fluoxetine. Diazepam has a half-life of 20-100 hours with an active metabolite (desmethyldiazepam) with a half-life of 36-200 hours, which is why it accumulates dramatically in elderly patients.

Short half-lives and dosing frequency

A drug's half-life directly drives its dosing interval. Most drugs are dosed at intervals of one half-life or less to maintain therapeutic concentrations. Aspirin's antiplatelet effect lasts the life of the platelet (8-10 days) even though salicylate has a 15-20 minute half-life - an important exception to this principle. For patients managing complex multi-drug regimens, use our Medication Schedule Planner to organise dosing times. For drug interactions that affect metabolism and therefore effective half-life, use our Drug Interaction Checker.

Steady state and time to therapeutic effect

When you start a regular-dosing medication, drug accumulates until the amount absorbed with each dose equals the amount eliminated. This is steady state. It takes 4-5 half-lives to reach steady state. For a drug with a 12-hour half-life dosed twice daily, steady state is reached in about 2.5 days. For fluoxetine, it takes 4-6 weeks. This is why antidepressants take weeks to show full clinical effect.

Renal function and half-life extension

For renally-cleared drugs, reduced kidney function directly extends half-life, since the primary elimination pathway is slowed. A drug that normally clears in 4-5 half-lives can take substantially longer in a patient with impaired renal function, increasing accumulation and toxicity risk if dosing isn't adjusted. Use our Creatinine Clearance Calculator to assess renal function before relying on standard half-life figures for dosing decisions.

Frequently asked questions

Drug half-life (t½) is the time required for the concentration of a drug in the body to reduce by 50%. After 1 half-life, 50% remains. After 5 half-lives, approximately 97% has been eliminated and the drug is considered clinically gone for most purposes.
4-5 half-lives is the clinical standard for "essentially eliminated" (less than 3-6% remaining). For fluoxetine with a half-life up to 6 days: up to 30 days. For aspirin: about 1.5 hours. For diazepam with a half-life up to 100 hours: up to 3 weeks for the parent drug alone, not counting active metabolites.
Steady state is reached after 4-5 half-lives of regular dosing. At steady state, the amount absorbed with each dose equals the amount eliminated. For a drug with a 12-hour half-life: about 2.5 days. For fluoxetine with a half-life of several days: 4-6 weeks.
Yes, significantly. Renal disease extends the half-life of renally-cleared drugs (gentamicin, vancomycin, metformin, digoxin) by reducing elimination. Liver disease extends the half-life of hepatically-metabolised drugs by reducing metabolic capacity. This is why dose adjustments are needed in these patient groups. Use our Renal Dose Adjustment Calculator for specific guidance.