When opioids bind to their receptors, pain transmission is reduced (Baaklini, Arruda, & Sakata, 2017; Nechifor, 2012), but the sensitivity to pain may increase (Angst & Clark, 2006; Baaklini et al., 2017; DuPen, Shen, & Ersek, 2007). Magnesium plays a role in pain sensitivity (Baaklini et al., 2017; Célèrier et al., 2000) and has been studied in combination with opioids for cancer-related pain. 

Angst, M.S., & Clark, J.D. (2006). Opioid-induced hyperalgesia: A qualitative systematic review. Anesthesiology, 104, 570-587.​ Retrieved from http://anesthesiology.pubs.asahq.org/article.aspx?articleid=1923441 

Baaklini, L.G., Arruda, G.V., & Sakata, R.K. (2017). Assessment of the analgesic effect of magnesium and morphine in combination in patients with cancer pain: A comparative randomized double-blind study. American Journal of Hospice and Palliative Medicine, 34, 353–357. https://doi.org/10.1177/1049909115621895  

Célèrier, E., Rivat, C., Jun, Y., Laulin, J.P., Larcher, A., Reynier, P., & Simonnet, G. (2000). Long-lasting hyperalgesia induced by fentanyl in rats preventive effect of ketamine. Anesthesiology, 92, 465-465.​ Retrieved from http://anesthesiology.pubs.asahq.org/article.aspx?articleid=1946090

DuPen, A., Shen, D., & Ersek, M. (2007). Mechanisms of opioid-induced tolerance and hyperalgesia. Pain Management Nursing, 8, 113-121. https://doi.org/10.1016/j.pmn.2007.02.004

Nechifor, M. (2012). Magnesium involvement in pain. Magnesium Research, 24, 220-222.​