Caffeine and the closely related alkaloids theobromine and theophylline make up the primary active constituents in guaraná. Caffeine’s effects are well known and include stimulating the central nervous system, increasing metabolic rate, and having a mild diuretic effect. One preliminary trial found no significant actions on thinking or mental function in humans taking guaraná. Guaraná also contains tannins, which act as astringents and may prevent diarrhea. However, this action has not been studied in human clinical trials.