DISCRIMINATIVE STIMULUS AND SUBJECTIVE EFFECTS OF THEOBROMINE AND CAFFEINE IN HUMANS

Theobromine versus placebo discrimination and caffeine versus placebo discrimination were studied in two consecutive experiments in seven vo lunteers who abstained from methylxanthines. Daily sessions involved P O double-blind ingestion of two sets of capsules sequentially, one of which contained drug and the other placebo. Subjects attempted to iden tify, and were later informed, which set of capsules contained the dru g. In each experiment subjects were exposed to progressively lower dos es. Five subjects acquired the theobromine discrimination; the lowest dose discriminated ranged from 100 to 560 mg. All seven subjects acqui red the caffeine discrimination; the lowest dose discriminated ranged from 1.8 to 178 mg. A final experiment evaluated subjective effect rat ings following 560 mg theobromine, 178 mg caffeine and placebo, which were administered double-blind in capsules once daily, five times each in mixed sequence. Caffeine produced changes in both group and indivi dual ratings (e.g. increased well-being, energy, social disposition an d alert). Theobromine did not produce changes in group ratings but cha nged ratings in some subjects. Across subjects, sensitivity to caffein e discriminative effects in the discrimination experiment correlated s ignificantly with the number and magnitude of caffeine subjective effe cts in the final experiment. This study documents modest discriminativ e effects of theobromine in humans, but the basis of the discriminatio n is unclear. This study suggests that commonly consumed cocoa product s contain behaviorally active doses of caffeine and possibly theobromi ne.