Medication Half-Life and Clearance Calculator

MEDICAL DISCLAIMER

This calculator is for pharmacology education only. Never use this to adjust medication doses or timing without consulting your prescribing physician or pharmacist.

Drug Half-Life

The biological half-life (t½) is the time required for the drug concentration in the body to fall by 50%.
After 1 half-life: 50% of drug remains.
After 2 half-lives: 25% remains.
After 5 half-lives: about 3.1% remains, which is treated as clinically cleared.
Formula: C(t) = C₀ × (0.5)^(t/t½) = C₀ × e^(−0.693t/t½)

What this page answers, and what its sibling does

This page works from the half-life alone: when a regular dose reaches steady state, when a stopped drug is 90% and 97% gone, and the elimination rate constant. Add a dose and a time and it gives the level at that moment too. For one dose followed hour by hour, with the percentage left and the elimination timeline, the drug half-life calculator is the one built for it. Both use the same first-order model and agree to the decimal.

Common Drug Half-Lives

Very short, under 1 hour: adenosine (under 10 seconds), epinephrine (under 5 min), IV insulin (5-10 min)
Short, 1 to 6 hours: amoxicillin (1h), ibuprofen (2h), acetaminophen (2-3h), salicylate from low-dose aspirin (2-3h; aspirin itself lasts about 20 minutes)
Medium, 6 to 24 hours: lisinopril (12h), atorvastatin (14h)
Long, 1 to 3 days: sertraline (26h), digoxin (36-48h), fluoxetine (1-4 days)
Very long, weeks to months: amiodarone (about 58 days on average, anywhere from 15 to 142), chloroquine (1-2 months)

Norfluoxetine, the active metabolite of fluoxetine, runs 4 to 16 days on its own, which is why stopping fluoxetine produces so little withdrawal compared with other antidepressants and why it has to be cleared before starting an MAOI (monoamine oxidase inhibitor), which the label puts at 5 weeks.

Steady State

With regular dosing, levels reach steady state after about 4 to 5 half-lives. This page uses 4, which puts you at 93.75% of the eventual level.
Average steady-state concentration = (Dose × Bioavailability) / (Clearance × Dosing interval)
Peak: once the dose is absorbed, an hour or two after most tablets. Trough: just before the next dose.
Steady state has to sit inside the therapeutic window. Too low and nothing happens, too high and you get toxicity.

Loading Dose

To achieve steady-state concentrations immediately, a loading dose is sometimes given.
Loading dose = Target concentration × Volume of distribution
Common for drugs with long half-lives (digoxin, amiodarone, phenytoin).

Elimination Rate Constant

k_el = 0.693 / t½
C(t) = C₀ × e^(−k_el × t)
Volume of distribution (Vd) relates blood concentration to total body drug: Dose = Vd × C