Caffeine is a trimethylxanthine in which the three methyl groups are located at positions 1, 3, and 7. A purine alkaloid that occurs naturally in tea and coffee. It has a role as a central nervous system stimulant, an EC 3.1.4.* (phosphoric diester hydrolase) inhibitor, an adenosine receptor antagonist, an EC 2.7.11.1 (non-specific serine/threonine protein kinase) inhibitor, a ryanodine receptor agonist, a fungal metabolite, an adenosine A2A receptor antagonist, a psychotropic drug, a diuretic, a food additive, an adjuvant, a plant metabolite, an environmental contaminant, a xenobiotic, a human blood serum metabolite, a mouse metabolite, a geroprotector and a mutagen. It is a purine alkaloid and a trimethylxanthine.