Caffeine is a widely consumed alkaloid that is present in coffee, tea, coca products and cola drinks. It produces increased alertness, decreased sleep, insomnia and increased ability to work out cognitive problems. At low doses it produces an increased sense of well-being and mental capacity. With larger doses the irritability may develop into what is commonly known as `coffee nerves`. Caffeine is a CNS stimulator and acts as antagonist on adenosine receptors throughout the body. Caffeine undergoes extensive oxidative metabolism in humans and other mammalian species, initially by N-demethylation (which takes place almost exclusively in the liver) to produce theobromine, paraxanthine and theophylline and subsequently to 1,7-dimethylurate, 1-methylxanthine and 1-methylurate. All of these primary and secondary metabolites of caffeine are excreted in urine. N3 demethylation of caffeine reflects the activity of cytochrome P4501A2 (CYP1A2) enzyme which is responsible for the activation of numerous promutagens and procarcinogens such as aromatic amines and heterocyclic amines. Molecular modelling analyses show that caffeine and all its metabolites have large solvation energy values so that they can be excreted easily in the form of urine. Neither caffeine nor its metabolites has very small LUMO-HOMO energy difference. The high kinetic stability and high clearance rate for caffeine and all its metabolites may mean that none will be highly toxic. The metabolite AFMU has the smallest LUMO-HOMO energy difference (4.41 eV from DFT calculations) indicating that it will be somewhat more labile kinetically.