Do Opioids, such as Morphine, Affect the Immune System?
Answer: There is some evidence in the research literature that opioids can reduce the effectiveness of the immune system, and we have seen some clinical evidence of infections being treated by antibiotics that do not heal until the opioid dose is lowered. The suspected reason is that the opioid drugs might occupy the opiate receptors that naturally appear on the surface of immune system cells, thereby preventing their occupation by natural internal opiates (endorphins), which appear to promote healing. There are likely other ways that the opioid drugs interfere with the natural endorphin effects on enhancing immunity, too. However, the stress of intense pain also decreases immune system function, so if your pain is well relieved by an opioid prescribed to you, it may be better for you to get some pain relief as it may in turn help your immune system.
Source: Vallejo R, de Leon-Casasola O, Benyamin R. Opioid therapy and immunosuppression: a review. American Journal of Therapeutics. 2004; 11(5): 354-365. http://europepmc.org/abstract/MED/15356431