The endogenous opioid system is one of the most studied innate pain-relieving systems. This system consists of widely scattered neurons that produce three opioids: β-endorphin, the met- and leu-enkephalins, and the dynorphins. These opioids act as neurotransmitters and neuromodulators at three major classes of receptors, termed μ, δ, and κ, and produce analgesia. Like their endogenous counterparts, the opioid drugs, or opiates, act at these same receptors to produce both analgesia and undesirable side effects. This article examines some of the recent findings about the opioid system, including interactions with other neurotransmitters, the location and existence of receptor subtypes, and how this information drives the search for better analgesics. We also consider how an understanding of the opioid system affects clinical responses to opiate administration and what the future may hold for improved pain relief. The goal of this article is to assist clinicians to develop pharmacological interventions that better meet their patient’s analgesic needs.

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