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, and hallucinations [46]. In contrast to these negative findings, two independent studies observed decreased pruritus in women carrying the G allele of A118G after morphine treatment for post-caesarean section pain [38, 47]. A118G was also associated with reduced ability to focus when treated with oxycodone [27]. These studies suggest OPRM1 polymorphisms can alter the side effects of opioid medications. Substantially more research is required on this subject, however, since these pharmacogenetic effects may be specific to the particular genetic variant, opioid treatment, and side effect being studied.
