“Cannabinoid receptor agonists, such as delta-9-tetrahydrocannabinol (Δ9-THC), have antinociceptive effects and, are increasingly used to treat pain, and medications including cannabinoid receptor agonists are approved for use in humans.
Cannabinoid receptor agonists [e.g. Δ9-tetrahydrocannabinol (Δ9-THC)] enhance the antinociceptive effects of mu opioid receptor agonists, suggesting that combining cannabinoids with opioids would improve pain treatment.
…these results provide additional support for combining opioids with cannabinoids to treat pain.”
http://jpet.aspetjournals.org/content/early/2014/09/05/jpet.114.216648.long