What is caffeine's half-life?

In a healthy adult, caffeine's half-life — the time the body needs to eliminate half the dose — averages 4 to 6 hours, with a median around 5 h. It varies a lot with genetics (CYP1A2), smoking (×0.5), pregnancy (×3), oral contraceptives (×2) and certain medications. In newborns it can reach 80 h.

Caffeine's pharmacokinetics are well characterised. Near-complete intestinal absorption within 30-45 minutes, a plasma peak around the 45-minute mark, distribution to all tissues — central nervous system and placenta included — then hepatic metabolism. The main enzyme is CYP1A2 in the liver, responsible for roughly 95 % of clearance. The metabolites paraxanthine, theobromine and theophylline are then excreted by the kidneys.

The CYP1A2 gene is highly polymorphic: a common variant (CYP1A2*1F, rs762551) splits the human population in two. Fast metabolisers (AA genotype) clear caffeine in about 4 h, enjoy the alertness boost without evening interference, and handle 4-5 cups without trouble. Slow metabolisers (AC and CC genotypes) have a half-life of 6-8 h, accumulate caffeine through the day, and often see their sleep disrupted by a coffee after 2 p.m. According to several studies (JAMA 2006, Cornelis et al.), this polymorphism also modulates the cardiovascular risk of heavy intake in slow metabolisers.

Other factors move the needle. Smoking roughly doubles clearance (cigarette smoke induces CYP1A2), so a heavy smoker metabolises caffeine in 2-3 h and often drinks more to compensate; when they quit, the half-life nearly doubles again — a frequent cause of insomnia during nicotine cessation. Pregnancy stretches the half-life to 10-15 h by the third trimester; oral contraceptives double it; certain drugs (fluvoxamine, ciprofloxacin, various SSRIs) lengthen it further. At the opposite end, newborns barely metabolise caffeine at all: a half-life up to 80 h explains the strict 200 mg/day limit during breastfeeding.

Practically, the 'five half-lives' rule gives the decay: after 5 × 5 h = 25 h only about 3 % of the dose remains. For a 130 mg espresso at 8 a.m., you still carry ~65 mg at 1 p.m., ~32 mg at 6 p.m., ~16 mg at 11 p.m. and ~8 mg at 4 a.m. — non-trivial for a slow metaboliser. That is the quantitative backbone of the 'no coffee after 2 p.m.' rule of thumb. This FAQ describes pharmacology; for personal medical questions (medication, pregnancy, liver disease), please see a healthcare professional.

Caffeine half-life by profile

Individual variation and why caffeine timing advice isn't universal

The caffeine half-life of 5–6 hours is a population average that conceals a wide individual distribution. In the fastest metabolisers — typically CYP1A2*1A homozygotes who are also non-smokers, non-oral-contraceptive users, and have normal liver function — caffeine half-life can fall to 2.5–3 hours, meaning an afternoon espresso at 3 PM has almost completely cleared by 11 PM. In the slowest metabolisers — including pregnant women (where half-life extends to 9–11 hours due to hormonal enzyme inhibition), people taking certain medications (fluoroquinolone antibiotics, some antidepressants), and slow-metaboliser genotypes — a 3 PM espresso may still have significant caffeine activity at 6 AM the following morning.

Smoking dramatically reduces caffeine half-life — smokers metabolise caffeine roughly twice as fast as non-smokers because tobacco compounds induce CYP1A2 enzyme activity. This explains why regular smokers often consume more coffee than non-smokers to achieve equivalent caffeine effects: their faster clearance rate means each dose becomes ineffective more quickly, requiring more frequent dosing. The clinical implication appears when smokers quit: within 1–3 weeks of cessation, CYP1A2 activity normalises, caffeine metabolism slows to the non-smoker rate, and previous coffee consumption habits can suddenly produce what feels like caffeine toxicity. Many smokers who quit report increased coffee sensitivity and need to reduce their coffee intake in the first months post-cessation for this pharmacokinetic reason.

Going deeper

The practical application of caffeine half-life knowledge is in coffee timing decisions. If you know your approximate metaboliser type — fast (caffeine sensitivity is low, you can drink coffee relatively late), slow (you notice sleep effects from afternoon coffee, you have a genotype associated with slow metabolism), or intermediate (the average case) — you can calculate your personal 'coffee cutoff time' rather than applying a universal rule. A fast metaboliser with a 3-hour half-life drinking a 100 mg caffeine espresso at 6 PM has about 12.5 mg remaining at 11 PM — negligible. A slow metaboliser with an 8-hour half-life drinking the same espresso at 6 PM has about 70 mg remaining at 11 PM — enough to measurably affect sleep architecture. Knowing which you are is the first step to making coffee timing decisions that actually match your biology.