Effect of dose on the ability of caffeine to serve as a reinforcer in humans.

This study tested the effects of dose on the reinforcing effects of caffeine in humans. Eight moderate coffee drinkers were given concurrent access to decaffeinated coffee vs. decaffeinated coffee to which different doses of caffeine (25, 50, 150 and 200mg/cup) were added. Subjects were tested across several independent double-blind trials. The coffees with 25mg of caffeine were repeatedly self-administered at a rate greater than that of decaffeinated coffee in two of six subjects, the 50mg dose in four of eight subjects, the 150mg dose in three of six subjects, and the 200mg dose in none of the three subjects tested. Headaches, drowsiness and fatigue occurred with use of decaffeinated coffee in five subjects. When these symptoms occurred, there was a greater probability of self-administration of the caffeinated coffee. We conclude that doses of caffeine similar to those in tea or soda can serve as reinforcers.