What this means

CYP1A2 is the liver enzyme that breaks down about 95% of the caffeine you consume. The rs762551 A>C variant in the gene's promoter region affects how strongly the gene gets switched on, particularly in response to inducers like tobacco smoke and cruciferous vegetables. AA carriers are commonly labelled "fast" metabolisers, with shorter caffeine half-lives; CC carriers process caffeine more slowly. The effect is measurable but modest, and behaviour (timing of coffee, total dose, sleep debt) usually dominates how caffeine actually affects you.

Your liver breaks down about 95% of the caffeine you drink using a single enzyme called CYP1A2. A common DNA change affects how strongly this gene gets switched on, especially in response to things like cigarette smoke and broccoli. People with the "fast" version clear caffeine quicker; people with the "slow" version hold onto it longer. The difference is real but modest — usually a couple of hours in half-life either way. Timing of your coffee, total dose, and how rested you are matter more than which version you carry.

Caveats

• Effect on caffeine half-life is modest — usually a couple of hours either way.
• Tobacco smoking induces CYP1A2 sharply and can swamp the genotype effect.
• Many other factors shape caffeine response (sleep, adenosine receptor variants, tolerance).
• The "fast/slow" label is a continuum, not a clean binary.

References

• Sachse et al. — Functional significance of a C→A polymorphism in intron 1 of the cytochrome P450 CYP1A2 gene tested with caffeine (British Journal of Clinical Pharmacology, 1999)
• Cornelis et al. — Coffee, CYP1A2 genotype, and risk of myocardial infarction (JAMA, 2006)