According to a study conducted by researches of Harvard Medical School in Boston, THC (dronabinol) was able to reduce pain in 30 patients taking opioids for chronic pain.

Phase I of this 2-phase study was a double-blinded trial in which subjects were administered once on three occasions either 10mg or 20 mg of THC or identical placebo capsules. Phase II was an open-label individually dosed trial of THC as add-on medication to patients on stable doses of opioids.

Results of the Phase I study showed that patients who received THC experienced decreased pain intensity compared with placebo. No differences in benefit were found between the two THC doses. In the Phase II trial, titrated THC contributed to significant relief of pain and increased satisfaction compared with baseline. The incidence of side effects was dose-related. Overall, the use of THC was found to result in additional analgesia among patients taking opioids for chronic non-cancer pain.

Source: Narang S, Gibson D, Wasan AD, Ross EL, Michna E, Nedeljkovic SS, Jamison RN. Efficacy of Dronabinol as an Adjuvant Treatment for Chronic Pain Patients on Opioid Therapy, Journal of Pain, Dec 12, 2007 -
http://www.ncbi.nlm.nih.gov/pubmed/18088560