As can be clearly seen, when caffeine is metabolised in the body, it undergoes demethylation, at the hexane 2 position, to reach theobromine.

Caffeine is an alkaloid which can be obtained from the wastes of tea, or coffee or from the dried leaves of Camellia sinesis, or it may be prepared synthetically. It is also present in guarana, maté and kola. Both caffeine and theobromine are naturally occurring members of the methylxanthine family and whilst caffeine has an effect on predominately the central nervous system, theobromine is associated with diuretic responses. Methylxanthines are not particularly potent, needing to be administered in gram doses.

Caffeine can kill. Although this is not often realised, it does have a toxicity level, which varies from person to person but is typically in the region 5 to 50 g. For comparison, a typical cup of coffee contains approximately 100 mg of caffeine. This would mean needing to drink at least fifty cups of coffee in one go for it to be toxic. It should also be noted that these are the toxicity levels for humans. The levels for horses could be vastly different. Although it is not easy to say whether they could ingest more or less, as their body weight is much greater, this might imply the ability to ingest more. Caffeine is not a regular part of a horses diet where as for humans it is. Therefore, this suggests humans may be more tolerant of caffeine.