Regard of functional pharmacogenetic polymorphisms may further the success of pain therapy by adopting individualized approaches. The mu-opioid receptor gene (OPRM1) 118A>G polymorphism is a promising candidate for both opioid effects and pain because of both biological reasonability and apparent experimental and clinical evidence. We analyzed its importance for pain therapy using a meta-analytic approach to studies relating it to opioid pain therapy. Data from suitable studies selected from hits of a PubMed search for "OPRM1" were independently extracted by two authors. The meta-analysis included phenotypes by OPRM1 genotype (opioid dosing, pain, and side effects), publication year, diagnostic status, proportion of male study participants, and whether genotype frequencies agreed with Hardy-Weinberg equilibrium. We found no consistent association between OPRM1 118A>G genotypes and most of the phenotypes in a heterogeneous set of eight clinical studies. Only weak evidence of an association with less nausea (effect size, Cohen's d=-0.21, p=0.037) and of increased opioid dosage requirements (d=0.56, p=0.018) in homozygous carriers of the G allele was obtained. This indicates that despite initially promising results, available evidence of the clinical relevance of the OPRM1 118A>G polymorphism does not withhold a meta-analysis. This discourages basing personalized therapeutic concepts of pain therapy on OPRM1 118A>G genotyping at the present state of evidence.