Although smoking "moderate" doses of cannabis helps reduce pain in healthy research volunteers, pain is actually increased at high doses, a new study shows.

The November issue of the journal Anesthesiology features a study showing that marijuana, or cannabis, reduces pain only within a smaller dose range.

"Our study suggests that there is a therapeutic window for analgesia, with low doses being ineffective, medium doses resulting in pain relief and high doses increasing pain," said lead author Dr. Mark S. Wallace of University of California, San Diego, in a news release.

Wallace and colleagues evaluated the effects of smoking marijuana on pain responses in 15 healthy volunteers. On different days, the research subjects smoked low, medium or high doses of cannabis (based on the content of 9-delta-tetrahydrocannabinol, the main active chemical in marijuana), or an inactive placebo.

Pain was induced by injecting capsaicin, the "hot" chemical found in chili peppers, into the skin. Capsaicin injection is a standard technique used in pain studies, according to the authors.

Five minutes after smoking, none of the three doses of cannabis had any effect on pain responses to capsaicin.

However, 45 minutes after smoking the moderate dose of cannabis, pain was significantly reduced — approximately six points lower on a 100-point scale, compared with the inactive placebo.

In contrast, 45 minutes after smoking the high dose of cannabis, pain scores were increased — about eight points higher than with placebo.

The low dose of cannabis had no effect on pain scores. None of the three doses affected the spread of pain beyond the area injected with capsaicin.

Levels of THC measured in the blood were significantly related to reduced pain scores at the moderate dose of cannabis, but not to the increase in pain with high-dose cannabis. The volunteers' sense of feeling "high" increased with each dose of cannabis, even though the pain-relieving effects did not.

"With several states having passed laws legalizing the medical use of cannabis, there has been a call for more research on medicinal cannabis," said Wallace.

Wallace and colleagues call for further research, including studies of the clinical value of the pain-reducing effects. Based on this studies findings, Wallace said he would not recommend marijuana as a method of pain reduction at this time.