Factors influencing self-administration of, and subjective response to, placebo marijuana

Self-administration of, and subjective response to, placebo marijuana were studied in two groups of regular marijuana smokers. One group received the drug under a set of instructions that informed them that the marijuana was active (deceptive administration); the other group was informed that the marijuana might be inactive (double-blind administration). Subjects were allowed to smoke placebo marijuana freely for 60min during four identical weekly sessions. Subjects smoked an average of 6.0 half-length cigarettes per session, resulting in a mean increase in expired air carbon monoxide of 14.6 ppm. Placebo self-administration did not change significantly across the four sessions. Smoking was associated with marijuana-like subjective effects. Subjects in the deceptive administration group smoked more placebo marijuana and reported a greater subjective response than the other group during the first session only. Several anamnestic factors (drug use history, current pattern of marijuana use, dimensions of personality) correlated with the amount of placebo self-administered, and subjects with less marijuana experience tended to report stronger subjective responses to the placebo. These results demonstrate the importance of including a placebo control when studying the reinforcing effects of marijuana and identify some factors that might predict placebo responses to marijuana or other drugs.