Interview style article. Here is the key takeaway:

RG: What did you find?

Nielsen: We found there were quite a few animal studies (nineteen in total) that examined the synergistic effects of opioids and cannabinoids, and almost all of these confirmed that by combing opioids and cannabinoids, much lower doses of opioids are needed for effective pain relief. For example, the dose of morphine needed was four times lower when cannabinoids were given at the same time, and for codeine it was ten times lower. While this is very powerful, we often find that results in humans are quite different from those seen in animals.