The dual effect of morphine on tumor development

Morphine is a classic opioid drug used for reducing pain and is commonly prescribed as an effective drug to control cancer pain. Morphine has a direct role in the central nervous system to relieve pain, but because of its peripheral functions, morphine also has some side effects, such as nausea, constipation, and addiction (Gupta et al. in Sci World J 2015:10, 2015). In addition to its analgesic effect, the role of morphine in tumor development is an important question that has been investigated for many years with conflicting results. Numerous studies suggest that morphine has a role in both promoting and inhibiting tumor growth. In this extensive review, we attempt to comprehensively understand the effects of morphine and summarize both its positive and negative influences on various aspects of tumors, including tumor growth, angiogenesis, metastasis, inflammation, and immunomodulation.