Effects of moderate and high doses of marihuana on thermal pain: a sensory decision theory analysis.

Sixteen habitual marihuana users, selected for their good mental and physical health, were hospitalized for three months in the New York State Psychiatric Institute. During the first month, the subjects were drug free; during the second month, they smoked marihuana cigarettes provided by NIDA (2% THC, 20 mg per cigarette) at the rate of 3 to 12 a day. A modified Hardy-Wolff dolorimeter was used to present 20 thermal stimuli of 30-second duration in a random manner at nine different intensities. Subjects responded from a 14-category scale, and data were analyzed according to sensory decision theory analysis. During the third month, the subjects were again drug free. At the noxious thermal intensities, there was a decrease in the pain report criterion during the first two weeks of smoking. The pain enhancement effect was followed by return to the presmoking pain level during weeks 3 and 4 and in the postsmoking period, indicating that tolerance had developed. There was also an increase in pain discriminability during the four weeks of smoking which extended for one week after smoking. Tolerance developed to the pain report criterion but not to the thermal discriminability. This study suggests that marihuana may have hyperalgesic activity and probably enhances the perception of pain, in moderate smokers. In contrast, heavy smoking had little effect on discriminability and caused an increase in the pain report criterion.