Methylxanthines: toxicity to humans. 3. Theobromine, paraxanthine and the combined effects of methylxanthines.

This review provides a brief overview of known information on the human toxicity of theobromine and paraxanthine. Theobromine has some pharmacological effects, although these activities are considerably weaker than those of theophylline and/or caffeine, described in parts 1 and 2 of this series (Stavric, Fd Chem. Toxic. 1988, 26, 541 & 645). Paraxanthine, which is not found in plants or foods, is the major metabolite of caffeine in humans, in whom its toxicological potency appears to be very low. This paper gives a brief retrospective view of possible toxicological effects when methylxanthines are taken simultaneously or are present in combination as a result of metabolic transformation. Critical review of toxic manifestations due to exposure to relatively large doses of caffeine and theophylline indicates that such combined exposure may potentiate the toxic effects of either drug.