Opioids remain central to managing moderate to severe pain, yet they also produce significant and often under-recognized effects on the immune system. In this narrative review, we synthesize evidence from 1994 to 2025 across preclinical, translational, observational, and limited interventional studies in adults to examine how different opioid classes modulate immunity and the clinical relevance of these effects. Opioids act directly on immune cells via mu-opioid receptors (MORs), nociceptin/orphanin FQ receptors (NOR), and Toll-like receptor 4 (TLR4), and indirectly through neuroendocrine, autonomic, neuroinflammatory, and gut microbiota-mediated pathways. Immunologic consequences are drug specific: Morphine, fentanyl, and to a lesser extent methadone exhibit pronounced immunosuppressive profiles; oxycodone appears comparatively less suppressive; and buprenorphine and tramadol generally preserve, and may in some contexts enhance, immune function. Clinically, chronic or intensive opioid exposure is associated with increased risk of infection and sepsis-related mortality, potential facilitation of tumor progression or recurrence, impaired perioperative and transplant outcomes, and contributions to tolerance and opioid-induced hyperalgesia, with convergent data indicating these immune effects are intrinsic to opioid pharmacology. Framing opioid-induced immunomodulation as a clinically meaningful, agent-specific phenomenon argues for incorporating immunologic risk into analgesic selection—prioritizing less immunosuppressive opioids where appropriate, considering rotation and tapering strategies, using peripherally acting antagonists, and implementing multimodal analgesia—while underscoring the need for standardized immunologic endpoints, rigorously controlled clinical studies, and development of next-generation analgesics that maintain effective pain relief while minimizing detrimental immune effects.
Voiculescu et al. (Tue,) studied this question.