This is a 2015 study out of Canada, published in The Journal of Pain by the American Pain Society. It was a prospective cohort study. They gave cannabis to patients for 1 year with a matched control being patients in the same clinic not using cannabis for chronic pain. 431 patients were enrolled in the study. The mean daily cannabis use was 2.5g/day of 12.5% potency.
There was no difference in the risk of serious adverse events
Importantly, 56% of these patients in chronic pain were taking opioids also.
This study suggests that, yes, you CAN take opioids and cannabinoids together without adverse events.
The Journal of Pain
Ware, MA, Wang, T, Shapiro, S, Collet, JP, 2015. Cannabis for the Management of Pain: Assessment of Safety Study (COMPASS). The Journal of Pain, 16(12), 1233-1242.