A summary of the study is available here.
Here is an excerpt:
The study used capsaicin, an alkaloid derived from hot chili peppers that is an irritant to the skin, to mimic the type of neuropathic pain experienced by patients with HIV/AIDS, diabetes or shingles – brief, intense pain following by a longer-lasting secondary pain. The subjects were healthy volunteers who inhaled either medical cannabis or a placebo after pain was induced. The marijuana cigarettes were formulated under NIH supervision to contain either zero, two, four or eight percent delta-9-tetrahydrocannabinol (THC.)
“Subjects reported a decrease in pain at the medium dose, and there was also a significant correlation between plasma levels of THC, the active ingredient in cannabis, and decreased pain,” said Igor Grant, M.D., F.R.C.P.(C), professor and Executive Vice-Chair of the Department of Psychiatry, the director of the CMCR. “Interestingly, the analgesic effect wasn’t immediate; it took about 45 minutes for the cannabis to have an impact on the pain,” he said.