OPRM1 encodes the mu-opioid receptor (MOR), a G-protein coupled receptor that mediates analgesia and controls rewarding effects of opioids and other drugs of abuse 1. The receptor regulates pain perception through binding agonists like morphine and beta-endorphin, and can modulate endogenous pain inhibition through descending pain pathways 2. OPRM1 has multiple functional isoforms; some lack agonist-binding capacity but modulate signaling through oligomerization with binding-competent variants. Pharmacogenetically, OPRM1 polymorphisms—particularly the A118G variant—significantly influence opioid response. G-allele carriers require higher postoperative opioid doses than AA homozygotes, with effects most robust in Asian populations and morphine users 3. The G-allele associates with decreased conditioned pain modulation, potentially through reduced receptor expression or binding affinity 2. OPRM1 variants also contribute to opioid use disorder and alcohol dependence risk, with ethnicity-specific associations, though they explain only a modest portion of heritability 45. Clinically, the Clinical Pharmacogenetics Implementation Consortium provides CYP2D6-guided recommendations for codeine and tramadol therapy, though evidence for OPRM1-based clinical dosing remains limited for most opioids 67. OPRM1 genotyping may help predict individual analgesic requirements in select populations undergoing surgery.