Opioid receptors are intrinsically linked to a variety of diseases and their respective treatments, either as direct drug targets or through downstream signaling activated by endogenous opioids. MOR naturally regulates the analgesic response to pain and also controls the rewarding effects of many drugs of abuse, including opioids, nicotine, and alcohol. Because of the opioid receptor's involvement, many analgesics are direct MOR agonists and treatments for addiction often act as either agonists, partial agonists, or antagonists of MOR. The connection between MOR and both addiction and pain makes OPRM1, the gene encoding MOR, an interesting target for pharmacogenetic studies (Figure 1). Genetic variants in OPRM1, particularly A118G, have been repeatedly associated with the efficacy of treatments for pain and alcohol dependence. In the two most well replicated findings, patients carrying the G allele had a reduced analgesic response to exogenous opioids and alcoholics with the G allele had reduced relapse rates when treated with naltrexone. Additional connections between OPRM1 and treatments for opioid and nicotine addiction are also promising, but require further study. Clear definitions of the phenotypes and ethnicities
