“Cannabinoids and opioids have been shown to possess several similar pharmacological effects, including analgesia
The analgesic effects of opioids, such as morphine and codeine, in mice are enhanced by oral administration of the cannabinoid 9 -tetrahydrocannabinol (9 -THC).
These findings suggest that the use of a low-dose combination of analgesics is a valid and effective approach for the treatment of pain and necessitates further study.
In summary, we have observed that 9 -THC enhances the antinociceptive effects of morphine and codeine in a synergistic fashion. This is the first report of a true synergistic interaction between oral 9 -THC and morphine or codeine, since previous studies have only examined one-dose combinations.
Much more work needs to be done to elucidate the mechanisms by which cannabinoids and opioids interact to produce analgesia. However, the implication that a combination of drugs may be more effective than either drug alone, and at the same time possibly reduce the occurrence of side effects, should provoke further study on analgesic drug interactions.”
http://jpet.aspetjournals.org/content/jpet/304/3/1010.full.pdf