Caffeine self-administration in humans: 2. A within-subjects comparison of coffee and cola vehicles

The reinforcing effects of caffeine ingested in coffee versus cola were studied. Eleven participants who drank both coffee and cola (3-10 cups of coffee and 1-6 cans of cola daily; M = 632 mg caffeine/day) were tested in 4 conditions: cola at 33 mg/serving (the usual dose), coffee at 33 mg/serving, cola at 100 mg/serving, and coffee at 100 mg/serving (the usual dose). Each condition consisted of 6 double-blind weekly trials. In each trial, participants sampled caffeinated and noncaffeinated beverages and then had concurrent access to the 2 beverages. Relative use of these beverages was used to assess caffeine reinforcement. Across the 4 conditions (24 weeks), reliable caffeine reinforcement occurred in 5 participants (45%). Caffeine reinforcement did not differ as a function of vehicle or serving dose, and no vehicle-dose interactions were found. With both cola and coffee at the commonly used doses, self-reported motivation to work was greater and drowsiness and laziness smaller with caffeinated than noncaffeinated beverages. Results indicate that, among users of both coffee and cola, caffeine self-administration and subjective effects occur with both vehicles.