FACTORS INFLUENCING MARIJUANA SELF-ADMINISTRATION BY HUMANS

The self-administration of marijuana cigarettes varying in tetrahydroc annabinol (THC) content was measured by having participants choose bet ween marijuana and an alternative reinforcer, i.e., snack food. Twelve marijuana users (eight men, four women), in groups of four, participa ted in a 16-day residential study. Each day, participants had the oppo rtunity 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. D ays were divided into a work period (09.15-16.45 h), comprising perfor mance and subjective-effects tasks, and a recreation period (17.15-23. 30 h). Each day at 10.00 h, participants ''sampled'' a marijuana cigar ette 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 o nly other factor influencing marijuana choice was the opportunity to e arn additional money, with participants choosing not to smoke immediat ely before the math task. By the end of the study, active marijuana ha d smaller effects on ratings of ''high'', ''stimulated,'' and ''good d rug effect.'' These data demonstrate that: (a) Delta(9)-THC is an esse ntial reinforcing component of marijuana; (b) marijuana use may be man ipulated by monetary contingencies; and (c) tolerance may develop more readily to marijuana's subjective effects than its reinforcing effect s.