What is caffeine's half-life?
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About 5 hours on average, which means half of the caffeine you’ve absorbed is gone 5 hours later. In healthy adults it runs anywhere from 1.5 to 9.5 hours. The pill, pregnancy and smoking move it the most.
The 5 hour number gets quoted as if it were everyone’s. It’s the middle of a range where one person’s half-life can be six times another’s. Two people can drink the same 3pm coffee and go to bed with very different amounts still in them.
We’d treat your half-life as a range until your own nights tell you more. Start on 5 hours, move it if one of the things below applies to you, then test it for two weeks.
What a half-life means
Each half-life takes away half of what’s left. A 200 mg CELSIUS is down to about 100 mg 5 hours after it’s absorbed, 50 mg after 10 hours and 25 mg after 15. The amount keeps halving, so it fades slowly and never drops off a cliff. A small cup reaches a low level sooner than a big one, because it has fewer halvings to go.
What changes it
| Who | Half-life | The study |
|---|---|---|
| A typical adult | about 5 hours, anywhere from 1.5 to 9.5 | 2024 review |
| On a low-dose estrogen pill | 7.88 hours, against 5.37 in matched women who weren’t | 1985, women on the pill for over 3 months and 9 who weren’t |
| The last 4 weeks of pregnancy | 10.5 hours, against 3 in women who weren’t pregnant | 1982 |
| Pregnant, 38 to 40 weeks | clearance at 39% of the usual adult value | 15 women, 1982 |
| A heavy smoker, 20 a day or more | about a third shorter. Clearance fell 36% when they quit | 12 smokers, 2004 |
| Taking fluvoxamine | from 5 hours to 31 | 8 volunteers, 1996 |
A 2021 analysis that pooled data from 141 studies found the same two big effects held up over decades of research, with smoking speeding caffeine up and the pill slowing it down.
One coffee, four people
The chart below draws one 8 oz coffee at 2pm on four half-lives. At an 11pm bedtime, a heavy smoker (about 3.2 hours) has about 16 mg left. A typical adult (5 hours) has about 30. Someone on the pill (about 7.3 hours) has about 44, and someone late in pregnancy (10.5 hours) about 55. That’s more than three times as much, from the same cup.
What about genes?
DNA tests that cover caffeine look at a variant in CYP1A2, the liver enzyme that does most of the clearing, and call you a fast or slow metabolizer. In a 2025 trial, that variant and one in the adenosine receptor gene made no measurable difference to how caffeine affected sleep. It was a small trial of 23 men, so a small gene effect could have been missed (am I a slow caffeine metabolizer?).
Set your own
The half-life calculator moves the curve for the pill, pregnancy or smoking and shows how far your cutoff moves. The quiz folds it into your own cutoff time.
One coffee at 2pm, four people
- A heavy smoker, 3.2 hours: 16 mg at 11pm
- Typical, 5 hours: 30 mg at 11pm
- On the pill, 7.3 hours: 44 mg at 11pm
- Late in pregnancy, 10.5 hours: 55 mg at 11pm
8 sources
- Antonio J and others, Common questions and misconceptions about caffeine supplementation: what does the scientific evidence really show? Journal of the International Society of Sports Nutrition, 2024doi.org
- Abernethy DR, Todd EL, Impairment of caffeine clearance by chronic use of low-dose oestrogen-containing oral contraceptives, European Journal of Clinical Pharmacology, 1985pubmed.ncbi.nlm.nih.gov
- Knutti R, Rothweiler H, Schlatter C, The effect of pregnancy on the pharmacokinetics of caffeine, Archives of Toxicology Supplement, 1982pubmed.ncbi.nlm.nih.gov
- Parsons WD, Pelletier JG, Delayed elimination of caffeine by women in the last 2 weeks of pregnancy, Canadian Medical Association Journal, 1982pubmed.ncbi.nlm.nih.gov
- Faber MS, Fuhr U, Time response of cytochrome P450 1A2 activity on cessation of heavy smoking, Clinical Pharmacology and Therapeutics, 2004pubmed.ncbi.nlm.nih.gov
- Jeppesen U and others, A fluvoxamine-caffeine interaction study, Pharmacogenetics, 1996pubmed.ncbi.nlm.nih.gov
- Grzegorzewski J and others, Pharmacokinetics of Caffeine: A Systematic Analysis of Reported Data, Frontiers in Pharmacology, 2021doi.org
- Gardiner C and others, Dose and timing effects of caffeine on subsequent sleep: a randomized clinical crossover trial, Sleep, 2025doi.org
