There are numerous medical journals addressing this issue as well, however they require subscriptions. I am currently contacting one publisher to try to provide you with more information.

Please Copy And Paste To Your Browser:

https://www.google.com/amp/s/amp.cancer.org/treatment/treatments-and-side-effects/physical-side-effects/pain/opioid-pain-medicines-for-cancer-pain.html

From the article (which includes the safe use of opioids)

Opioids (narcotics) are used with or without non-opioids to treat moderate to severe pain. They are often a necessary part of a pain relief plan for cancer patients. These medicines are much like natural substances (called endorphins) made by the body to control pain. They were once made from the opium poppy, but today many are man-made in a lab.