Researchers at the University of California, Davis report that news in the June edition of The Journal of Pain.
Participants visited the researchers' lab three times, where they smoked a marijuana cigarette made for research purposes, under the National Institute of Drug Abuse's supervision.
During the sessions, which were held at least three days apart, participants either smoked a pot cigarette containing a high dose of THC (marijuana's active ingredient), a lower dose of THC, or no THC.
Participants got specific instructions about when to light up, inhale, and exhale. They were supervised as they smoked and for two hours after that, and then taken home.
Before and after smoking their assigned cigarette, participants rated their pain. Their pain ratings dropped more after smoking the THC cigarettes than the placebo cigarette lacking THC. The higher dose and lower THC doses had comparable effects, which began to wear off an hour or two after they stopped smoking.
But participants had no change in their pain sensitivity to light touch or heat after smoking any of the cigarettes, the study shows.
Participants also took tests of mental skills including memory and coordination before and after each smoking session. Their scores, many of which were low to begin with, showed the steepest decline after smoking the high-dose THC cigarette, followed by the low-dose THC cigarette.
They urge "caution in the prescribing of medical marijuana for neuropathic pain," especially in light of the mental impact, and also in young patients, as pot use in adolescence "increases the risk of later schizophrenia-like psychoses, especially in genetically susceptible individuals."