Factors influencing marijuana self-administration by humans.

The self-administration of marijuana cigarettes varying in tetrahydrocannabinol (THC) content was measured by having participants choose between marijuana and an alternative reinforcer, i.e., snack food. Twelve marijuana users (eight men, four women), in groups of four, participated in a 16-day residential study. Each day, participants had the opportunity to choose repeatedly between a marijuana cigarette and a snack. The THC concentration of the cigarette changed each day (0.0, 2.2 or 3.9% delta 9-THC w/w), as did the number of snack items (one or two); each THC concentration was compared to each snack condition twice. Days were divided into a work period (09.15-16.45 h), comprising performance and subjective-effects tasks, and a recreation period (17.15-23.30 h). Each day at 10.00 h, participants "sampled" a marijuana cigarette containing the delta 9-THC concentration available that day, and selected the number of snack items available that day. Six "choice" trials occurred from 14.00-19.00 h, when participants responded under a modified progressive ratio schedule for either marijuana or snacks. At 18.15 h, participants could participate in a 10-min math task, in which each correct answer earned $1.00. Cigarettes containing 2.2 or 3.9% delta 9-THC were self-administered more often than placebo. The only other factor influencing marijuana choice was the opportunity to earn additional money, with participants choosing not to smoke immediately before the math task. By the end of the study, active marijuana had smaller effects on ratings of "high", "stimulated," and "good drug effect." These data demonstrate that: (a) delta 9-THC is an essential reinforcing component of marijuana; (b) marijuana use may be manipulated by monetary contingencies; and (c) tolerance may develop more readily to marijuana's subjective effects than its reinforcing effects.