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 (less than 3-6% remaining).
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 a 5-week washout period is required before starting an MAOI after 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.