rs1799971 — OPRM1 A118G

The Mu-Opioid Receptor Variant — Your Body's Response to Pain and Opioid Medications

The OPRM1 gene encodes the mu-opioid receptor, the primary target for morphine, fentanyl, and most prescription opioid painkillers¹¹ morphine, fentanyl, and most prescription opioid painkillers
The mu-opioid receptor is also where your body's natural pain-relief system — endorphins and enkephalins — exerts its effects. The A118G variant, also known as Asn40Asp or rs1799971, is a substitution where asparagine is replaced by aspartic acid at residue 40 of the receptor protein. This single amino acid change occurs at an N-glycosylation site at the extracellular domain of the receptor , altering how the receptor is assembled and how it functions.

The Mechanism

The G allele is associated with reduced receptor expression in vitro and in vivo, although the mechanism of reduced receptor expression is unclear . Neuroimaging studies have shown²² Neuroimaging studies have shown
Using PET scans to measure opioid receptor binding in living brains that

G carriers show an overall reduction of baseline mu-opioid receptor availability in regions implicated in pain and affective regulation including the anterior cingulate cortex, nucleus accumbens, and thalamus. The functional consequence is that people with the G allele typically have fewer or less responsive mu-opioid receptors available to respond to both endogenous opioids (like endorphins) and exogenous opioids (like morphine).

The G variant is remarkably common in East Asian populations — occurring at frequencies of 40-60% in Asia and moderate frequency (15%) in samples of European ancestry . This substantial population difference means the clinical impact of this variant varies dramatically across ethnic groups, with roughly half of East Asians carrying at least one copy compared to about a quarter of Europeans.

The Evidence

Pain Management and Opioid Analgesia: The most consistent finding is that G allele carriers require higher doses of certain opioids for adequate pain control.

A meta-analysis of 18 studies involving 4,607 participants found G carriers needed more postoperative opioid medication than AA homozygotes. A 2019 meta-analysis³³ A 2019 meta-analysis
Yu et al. examined cancer pain specifically found

G allele carriers required more opioid analgesia in cancer pain management .

Importantly, not all opioids are equally affected.

A prospective study of 222 cancer patients found that pain relief after opioid therapy did not differ among genotypes for tapentadol or methadone, whereas it was significantly smaller in G-allele carriers for hydromorphone, oxycodone, and fentanyl . This suggests that tapentadol and methadone may be more suitable than hydromorphone, oxycodone, and fentanyl for G-allele carriers due to their dual mechanism of action

— these drugs work partially through non-opioid pathways (norepinephrine reuptake inhibition for tapentadol, NMDA receptor antagonism for methadone) that bypass the mu-opioid receptor deficit.

Substance Dependence and Addiction: Paradoxically, while G carriers show reduced opioid receptor function, a meta-analysis of 25 datasets with over 28,000 European-ancestry subjects found the G allele showed modest protective effects (OR=0.90) against general substance dependence .

The G variant is now one of the few examples of a genetic factor that demonstrates a similar, general effect across multiple substances .

Naltrexone for Alcohol Use Disorder: The story with naltrexone — a mu-opioid receptor blocker used to treat alcohol use disorder — is complex and controversial. Early retrospective studies suggested G carriers responded better to naltrexone, but larger prospective trials and meta-analyses⁴⁴ larger prospective trials and meta-analyses
The most rigorous recent evidence have been disappointing.

From the evidence to date, it remains unclear whether the OPRM1 Asn40Asp polymorphism predicts naltrexone treatment response in alcohol use disorder . The 2024 CPIC guideline explicitly states there are no therapeutic recommendations for dosing opioids based on OPRM1 genotype (CPIC level C) .

Pain Sensitivity and Side Effects:

The G allele is associated with a reduced risk of postoperative vomiting when opioids are used, though effects on nausea, pruritus, and dizziness are inconsistent.

Practical Implications

If you carry one or two copies of the G allele, you may experience reduced pain relief from commonly prescribed opioid medications including morphine, fentanyl, oxycodone, and hydromorphone. This does not mean these medications won't work — but you may need higher doses than average, or you may find better success with alternative opioids like tapentadol or methadone that work through multiple mechanisms.

For postoperative or acute pain management, discuss your genotype with your anesthesiologist or pain management physician. They may opt for multimodal pain control strategies — combining opioids with non-opioid medications like acetaminophen, NSAIDs, or regional anesthesia techniques — to achieve adequate pain control without excessive opioid doses.

The evidence does not support using OPRM1 genotype to guide naltrexone treatment for alcohol use disorder at this time, though research continues. If you're considering naltrexone, response should be judged on clinical outcomes rather than genotype.

Interactions

The mu-opioid receptor does not function in isolation. Animal studies and some human evidence suggest interactions between OPRM1 and dopamine system genes (like COMT and DAT1) may influence both naltrexone response and addiction vulnerability, but these interactions remain under investigation and are not yet actionable for clinical use. The endogenous opioid system also interacts extensively with the stress response system, pain pathways, and reward circuitry throughout the brain.