Although environmental factors influence the responses to opioids, genetic background plays a significant role (13–16). OPRM1 is the major target of both endogenous and exogenous opiate and has been shown to mediate both baseline nociception and response to OPRM1 agonists (17,18). Both animal and human studies have indicated that reduced basal nociceptive sensitivity is associated with greater opioid analgesia (19–21) and suggested genetic polymorphisms in the human OPRM1 gene is the primary candidate source of clinically relevant variability in opiate sensitivity and baseline nociception (13–16).
