“Opioid receptor agonists are effective for treating pain; however, tolerance and dependence can develop with repeated treatment. Combining opioids with cannabinoids can enhance their analgesic potency…
These results demonstrate that antinociceptive tolerance is greater during treatment with the mixture, and although treatment conditions were sufficient for dependence to development on morphine, withdrawal was not markedly altered by concurrent treatment with THC.
Thus, THC can enhance some (antinociception, tolerance) but not all (dependence) effects of morphine.”