What biological half-life means
Biological half-life (t½) is the time it takes for the plasma concentration of a substance in the body (the amount of that substance in blood plasma) to fall to half of its starting value. It is not a chemical property of the molecule itself — it describes how quickly the body metabolizes and eliminates it.1
The starting value is most often peak plasma concentration (Cmax): the highest blood level reached after a dose.
From half-life to full elimination
Pharmacology uses a practical rule of thumb: after five half-lives, a substance is considered effectively eliminated — less than 3% remains in the body. This convention is used, for example, when switching between medicines or estimating a safety washout period.1
| Number of half-lives | Amount remaining | Caffeine (t½ ~5–6 h): time since one cup |
|---|---|---|
| 1× t½ | 50% | ~5 hours |
| 2× t½ | 25% | ~10 hours |
| 3× t½ | 12.5% | ~15 hours |
| 4× t½ | ~6% | ~20 hours |
| 5× t½ | ~3% | ~25 hours — effectively eliminated |
Everyday examples
Caffeine (t½ ~5–6 hours).2 A cup of coffee at 2:00 p.m. means that around 8:00 p.m., roughly half of that caffeine may still be in the blood. Full elimination takes about five half-lives — roughly 25–30 hours after the last dose.
Ibuprofen (t½ ~2 hours).3 Ibuprofen leaves plasma relatively quickly. To maintain an active amount in the blood, it needs repeated dosing — which is why common dosing instructions use intervals such as every 6–8 hours.
Semaglutide — Ozempic (t½ ~168 hours, ~7 days).4 This is the opposite extreme: a molecule intentionally designed with a long t½ so that one weekly injection is enough. After stopping semaglutide, the body effectively eliminates it in roughly 5 weeks.
A short t½ usually means more frequent dosing. A long t½ allows less frequent dosing — but it also means that if an adverse effect occurs, it may take longer to resolve.
Why t½ is always approximate
Values such as “~5 hours” or “~7 days” are population averages — measurements taken from groups of people in clinical studies and summarized statistically. Every body handles substances differently: body weight and composition, age, liver and kidney function, genetic enzyme variants (especially CYP450), and other medicines all matter.1,5
That is why t½ is usually written with a tilde (~) or as a range. A precise-looking number without uncertainty would be misleading. Your body is not the average body from a study — and that is normal, not an exception.
What biological half-life does not tell you
- It does not tell you how potent a substance is or when its maximum effect will occur.
- It does not tell you how long a substance feels active — half-life measures plasma concentration, not subjective experience. Caffeine is a useful example: you may stop consciously feeling the stimulant effect before caffeine has left the body, while it can still occupy adenosine receptors and affect sleep architecture.
- It does not tell you whether a substance is safe or effective at a given concentration.
If you want to understand when a stable blood level is reached after starting treatment, read about steady state.
Sources
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Brunton LL, Knollmann BC, Hilal-Dandan R, eds. Goodman & Gilman's: The Pharmacological Basis of Therapeutics. 14th ed. McGraw-Hill; 2023. Ch. 2: Pharmacokinetics: The Dynamics of Drug Absorption, Distribution, Metabolism, and Elimination. ↩ ↩2 ↩3
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Nehlig A, Daval JL, Debry G. Caffeine and the central nervous system: mechanisms of action, biochemical, metabolic and psychostimulant effects. Brain Res Brain Res Rev. 1992;17(2):139–170. PMID 1356551 ↩
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Davies NM. Clinical pharmacokinetics of ibuprofen. The first 30 years. Clin Pharmacokinet. 1998;34(2):101–154. PMID 9515184 ↩
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Lau J, et al. Discovery of the once-weekly glucagon-like peptide-1 (GLP-1) analogue semaglutide. J Med Chem. 2015;58(18):7370–7380. PMID 26308095 ↩
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Nehlig A. Interindividual differences in caffeine metabolism and factors driving caffeine consumption. Pharmacol Rev. 2018;70(2):384–411. PMID 29514871 ↩