Half-Life

Also known as: t½ · elimination half-life · biological half-life

Half-life (t½) is the time it takes for the concentration of a drug in your blood plasma to fall by half.

Half-life, written t½, describes how fast a drug's plasma concentration falls once absorption and distribution have settled. After one half-life, half the concentration is left; after two, a quarter; after three, an eighth. Because each half-life removes the same fraction rather than the same amount, the decline is exponential: about 97% is gone after five half-lives (1 − 0.5^5 = 0.969). Strictly, it is the time to halve the plasma concentration, not the time to eliminate half of the dose you injected.

Half-life is not a fixed property you can read in isolation. When absorption is not the slow step, it is set by two things: how fast your body clears the drug and how widely the drug spreads into tissues. When absorption from the injection site is slower than elimination (called flip-flop kinetics), the half-life you measure after an injection reflects the absorption rate instead of elimination. That is one reason a half-life from an intravenous study does not automatically describe the curve after a subcutaneous (SubQ) injection.

Why it matters

Half-life sets three things you will notice: how much levels swing between doses, how much a compound builds up with repeated doses, and how long it lingers after you stop. The semaglutide label gives an elimination half-life of about one week and states it will be present in circulation for about 5 weeks after the last dose, which is the five-half-life rule in practice.

For research peptides, published half-life numbers are scarce and often misquoted. The one published pharmacokinetic study of BPC-157 was done in rats and beagle dogs and found an elimination half-life for the intact peptide of under 30 minutes; it did not study people. Treat any precise human half-life you see for BPC-157, TB-500 or similar compounds as unsupported unless it cites a human study.

How to calculate it

Find a sourced half-life for the compound (prescribing information or a human pharmacokinetic study) and note the species and route it was measured in.
Divide the time elapsed by the half-life to get the number of half-lives, n.
Multiply the starting level by 0.5 raised to the power n to get what remains.
For the elimination rate constant, divide 0.693 (the natural log of 2) by the half-life.

C(t) = C0 × 0.5^(t / t½); k = 0.693 / t½

Worked example

Semaglutide's label half-life is about 1 week (168 hours). Starting from a level of 100 (any unit): after 1 week, 100 × 0.5 = 50; after 2 weeks, 25; after 3 weeks, 12.5; after 4 weeks, 6.25; after 5 weeks, 100 × 0.5^5 = 3.1. About 97% is gone, matching the label's 'about 5 weeks' after the last dose. The rate constant is k = 0.693 / 168 h ≈ 0.0041 per hour. You can plot the same curve for any sourced half-life with the /tools/half-life-decay calculator.

Common mistakes

Quoting a half-life with no source, or quoting a human half-life for a peptide that has only been studied in animals.
Assuming half the dose has left your body after one half-life; half-life tracks the fall in plasma concentration, and drug may still be absorbing from the injection site.
Using an intravenous half-life to plan SubQ timing when absorption is slower than elimination.
Thinking a compound is essentially gone after one or two half-lives; after two, 25% of the concentration remains.
Treating a multi-hour human half-life for BPC-157 as fact; the only published pharmacokinetic study measured under 30 minutes, in rats and dogs.

Sources

ReviewPlasma terminal half-life. J Vet Pharmacol Ther, 2004. PMID 15601438.
Prescribing informationOZEMPIC (semaglutide) injection, for subcutaneous use: Prescribing Information. Novo Nordisk (FDA-approved labeling), 2026.
Preclinical studyPharmacokinetics, distribution, metabolism, and excretion of body-protective compound 157, a potential drug for treating various wounds, in rats and dogs. Front Pharmacol, 2022. PMID 36588717.

Page updated October 3, 2026. How we grade evidence: methodology.

Related Terms

Cmax

Cmax is the highest concentration a drug reaches in your blood after a dose; Tmax is how long after the dose that peak occurs.

Cmin

Cmin is the lowest concentration of a drug in your blood during a dosing interval, usually reached just before the next dose (the trough).

Steady State

Steady state is the point in repeated dosing where the drug eliminated each interval equals the drug taken in, so levels stop climbing.

AUC (Area Under Curve)

AUC (area under the curve) is your total exposure to a drug over time: the area under its blood concentration-time curve.

Clearance Rate

Clearance (CL) is the volume of plasma your body fully clears of a drug per unit of time, usually given in liters per hour.

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