How long does caffeine last in your system?
Plasma caffeine peaks about 45 minutes after ingestion and has a half-life of roughly 5 hours, so a pre-race dose is still substantially present at the finish of a marathon.
Caffeine is absorbed quickly and almost completely from the gut, reaching peak plasma concentration around 45 minutes after ingestion. Elimination follows first-order kinetics with a half-life of approximately five hours in most adults, meaning half the dose remains after five hours and a quarter after ten.
For marathon running that timescale is convenient. Take 4 mg/kg an hour before the start and you are still carrying most of it at 30 km, when perceived exertion is highest and the effect matters most. There is no need to time a second dose to arrive at a particular point in the race.
The half-life varies considerably between people. Genetic differences in the CYP1A2 enzyme that metabolises caffeine produce fast and slow metabolisers with meaningfully different curves. Pregnancy and some medications extend it substantially; smoking shortens it.
The downstream consequence people underestimate is sleep. A five-hour half-life means an afternoon dose is still a quarter present at bedtime, and caffeine disrupts sleep architecture even when it does not prevent you falling asleep. For a runner in a heavy training block, that trade is often worse than the session it powered.
Key numbers
| Peak plasma | ~45 min | after ingestion |
|---|---|---|
| Half-life | ~5 hours | typical adult |
| Still present | ~25% at 10 hours |
Work it out for yourself
Caffeine & stimulants model — Plot the full absorption and elimination curve for your dose.
Frequently asked questions
When should I stop drinking coffee before bed?
Given a five-hour half-life, a dose at 3pm is still roughly a quarter present at 1am. Most sleep guidance suggests stopping eight to ten hours before bed, which is considerably earlier than most people assume.
Why does caffeine affect some people more than others?
Largely genetics. Variation in the CYP1A2 enzyme produces fast and slow metabolisers with substantially different elimination rates, and adenosine receptor differences affect sensitivity independently of metabolism.
Sources
- Southward, Rutherfurd-Markwick & Ali (2018). The effect of acute caffeine ingestion on endurance performance. Sports Medicine.
- Pickering & Kiely (2019). What should we do about habitual caffeine use in athletes? Sports Medicine.
Related explainers
Glossary: Caffeine, Pharmacokinetics (PK)