A recent review and meta-analysis of randomized clinical trials published in the BMJ has shown that medical cannabis can effectively alleviate pain in cancer patients. 
Pain related to a cancer diagnosis can stem from the disease itself or the treatment protocol. For instance, pain can be caused by the growth of cancerous tumors pressing on nearby nerves, organs, or bones. Cancer tumors may also release certain chemicals that cause pain. And while treatments for cancer — radiation, chemotherapy, and surgery — are meant to help curb the spread of the disease, they may also come with side effects, including pain and discomfort.
The study reviewed 32 trials with over 5,100 patients, who were either treated with medical cannabis or a placebo. Medical cannabis was administered orally or topically (not inhaled).
After following up with the participants between 1 to 5.5 months after the study, researchers found that medical cannabis led to very small to small increases in the number of patients experiencing pain alleviation. Cannabis also helped improve sleep quality and physical functioning.
However, researchers also found that medical cannabis taken orally does not improve emotional or social functioning. Further, there may be some evidence that medical cannabis taken orally may cause a small increased risk of vomiting, nausea, cognitive impairment, drowsiness, and dizziness compared to the placebo.
How Does Medical Cannabis Help Alleviate Cancer-Related Pain?
Cannabinoids including tetrahydrocannabinol (THC) and cannabidiol (CBD) may help relieve pain caused by cancer and other ailments by interacting with the cannabinoid receptors of the endocannabinoid system (ECS). By doing so, such cannabinoids can lower pain signaling and perception. 
THC interacts with the ECS by binding to both CB1 and CB2 receptors. CBD, on the other hand, does not bind directly to these receptors, but rather interacts indirectly with them. Activating the receptors of the ECS is what allows many of the health benefits to take place including pain relief.
While the consensus of this review was that only small changes in pain relief were recognized, medical cannabis provides a promising option for curbing discomfort caused by cancer and its treatments.
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1- Wang L, Hong PJ, May C, et al. Medical cannabis or cannabinoids for chronic non-cancer and cancer related pain: a systematic review and meta-analysis of randomised clinical trials. BMJ. 2021;374:n1034. [journal impact factor = 39.89; times cited = 8]
2- Russo EB. Cannabinoids in the management of difficult to treat pain. Ther Clin Risk Manag. 2008;4(1):245-259. [journal impact factor = 2.423; times cited = 197]