Caffeine pharmacokinetics

This page is a reference summary of published evidence and regulatory documents. It is not a substitute for individual medical advice, and it is not a recommendation to take or avoid any medicine.

Medically reviewed by Dr. Sofia Lindqvist, · Last reviewed 11 August 2026

Caffeine is absorbed almost completely from the gastrointestinal tract, reaching peak plasma concentration in 30-60 minutes. Its average half-life is approximately 5 hours, but ranges from about 2 to 10 hours between individuals depending on CYP1A2 enzyme activity and factors including smoking, oral contraceptive use and pregnancy.

Absorption and time to peak

Oral bioavailability of caffeine is close to 100%. After ingestion, plasma concentration typically peaks within 30-60 minutes, though peak timing can shift later with food intake, which slows gastric emptying without materially reducing total absorption.

CYP1A2 metabolism

Caffeine is metabolised primarily by the hepatic enzyme CYP1A2, which converts it mainly to paraxanthine along with smaller amounts of theobromine and theophylline. CYP1A2 activity varies substantially between individuals due to genetic polymorphisms (commonly discussed as “fast” versus “slow” metabolisers) and environmental factors that induce or inhibit the enzyme.

Factors that change half-life

Factor	Effect on half-life
Smoking	Roughly halves half-life (CYP1A2 induction)
Oral contraceptives	Roughly doubles half-life (CYP1A2 inhibition)
Pregnancy (third trimester)	Extends half-life to roughly 10-15 hours
Liver disease	Can substantially prolong half-life

Decay over time

Hours after dose	Approximate fraction remaining (5-hour half-life)
0	100%
5	50%
10	25%
15	12.5%
20	6.25%

Illustrative only; individual half-life ranges from roughly 2-10 hours.

A dose taken with a half-life of 5 hours still leaves roughly a quarter of its peak concentration in the body 10 hours later, which is relevant to caffeine taken in the afternoon or evening.

Frequently asked questions

How long does caffeine take to work?

Caffeine is absorbed almost completely from the gut, reaching peak plasma concentration (Tmax) in roughly 30-60 minutes.

What is caffeine's half-life?

Approximately 5 hours on average in healthy adults, with an individual range of roughly 2-10 hours depending on CYP1A2 activity and other factors.

Does smoking affect caffeine metabolism?

Yes. Smoking induces CYP1A2 and roughly halves caffeine’s half-life, so smokers clear caffeine faster than non-smokers.

Do oral contraceptives affect caffeine clearance?

Yes. Oral contraceptives inhibit CYP1A2 and roughly double caffeine’s half-life.

Does pregnancy change caffeine metabolism?

Yes. Caffeine clearance slows progressively through pregnancy, and half-life can extend to roughly 10-15 hours in the third trimester.