Opioid use disorder (OUD) is infrequent in AD; however, in AD cases treated with opioids, it is important to take into account that patients with at least one copy of the CYP3A4*1B allele exhibit an accelerated rate of metabolism compared to the wild-type allele CYP3A4*1 [339] and that CYP2D6-UMs show a better response to opiates than EMs, IMs, and PMs [55]. The gene discussed is CYP2D6; the disease is Alzheimer disease.