Both illicit and prescribed opioids have notable side effects including nausea, constipation, pruritus (itching), and respiratory depression. While pharmacogenetic studies often focus on treatment outcome, the effect of genetic variation on side effect severity is also relevant to appropriate treatment selection. Individuals with more severe side effects may be less likely to continue their prescribed treatment course, creating a link between the pharmacogenetics of side effects and treatment efficacy. These patients may also need additional treatment to manage the increased risk of side effects. Several studies have found no association between OPRM1 SNPs and opioid side effects. A118G did not have an effect on respiratory depression caused by morphine-6-glucuronide, a metabolite of morphine [22]. Nine SNPs in OPRM1 were found to not be associated with nausea and vomiting in cancer patients receiving opioids, including morphine, oxycodone, and fentanyl [45]. A similar lack of association was observed between A118G and nausea or vomiting due to fentanyl treatment in an additional study [33]. The intronic SNP rs2075572 was also only nominally associated with central side effects of morphine, such as nausea, drowsiness,
