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)

Common Drug Half-Lives Very short, under 1 hour: adenosine (6 sec), epinephrine (under 5 min), insulin (5-10 min) Short, 2 to 6 hours: ibuprofen (2h), acetaminophen (2-3h), aspirin (3-6h), amoxicillin (1h) 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 (40-55 days), 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.

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. Steady-state concentration = (Dose × Bioavailability) / (Clearance × Dosing interval) Peak: right after dose. Trough: just before 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