The rs1799971(G) allele in exon 1 of the mu opiod receptor OPRM1 gene causes the normal amino acid at residue 40, asparagine (Asn), to be replaced by aspartic acid (Asp). In the literature this SNP is also known as A118G, N40D, or Asn40Asp.
Carriers of at least one rs1799971(G) allele appear to have stronger cravings for alcohol than carriers of two rs1799971(A) alleles, and are thus hypothesized to be more at higher risk for alcoholism.
However, subsequent research results are mixed, and there are other studies both agreeing or disagreeing with this finding [PMID 15525999, PMID 9399694, PMID 12960749]
Among 200+ alcoholics treated with naltrexone, rs1799971(G) carriers receiving the drug (even without behavioral intervention) had an increased percentage of days abstinent (p = .07) and a decreased percentage of heavy drinking days (p = .04) if treated with naltrexone vs placebo, whereas rs1799971(A;A) homozygotes showed no medication differences. Upon treatment with naltrexone, 87% of rs1799971(G) carriers had a good clinical outcome, compared with only 55% of individuals with the (A;A) genotype (odds ratio, 5.75, CI: 1.88-17.54)[PMID 18250251]
This SNP may also influence the response to opioids such as heroin, codeine or morphine.
A study of 200 Chinese heroin addicts found increased frequency for the rs1799971(G) allele compared to non-addicts (40% vs 29%) [PMID 11338173]; but another study of Han Chinese addicts found no difference [PMID 11933204].
Based on a study at 207 patients treated with morphine, carriers of OPRM1 GG genotype required 93% higher morphine dose compared to carriers of AA genotypes (p = 0.012).
