Mechanism of Action of Opioid Drugs in Analgesia

Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, affecting nearly 20% of the global population; chronic pain constitutes approximately half of these cases. Therefore, the development of effective and safe analgesics is of paramount clinical importance. Opioid analgesics—encompassing natural, semi-synthetic, and synthetic agents—are widely used for moderate-to-severe pain. They primarily act through μ, κ, and δ opioid receptors and their downstream signaling pathways to produce analgesia. Despite their efficacy, clinical utility is constrained by adverse effects including opioid use disorder, tolerance, and opioid-induced hyperalgesia. This review summarizes opioid-mediated pain modulation in the central and peripheral nervous systems, emphasizing interactions with neural circuits, neurotransmitters, and immune cells. It further discusses clinical applications in conditions such as chronic pain, obstructive sleep apnea, and mild traumatic brain injury, and outlines strategies to mitigate side effects via targeted drug development, combination therapies, and alternative analgesic approaches.