Table 1.

Inclusion criteria for study selection in this meta-analysis

Number	Inclusion criteria
1	Original observational studies published in full text and those for which we had full access to all original data and protocols
2	The studies evaluated the associations between OPRM1 A118G polymorphism and postoperative side effects.
3	The studies included detailed genotyping data (total number of cases and controls, number of cases and controls with A/A, A/G, and G/G genotypes).
4	Studies focusing on human being.
5	predefined outcomes: incidence of postoperative nausea, vomiting, nausea and vomiting (PONV), pruritus, dizziness, et al.
6	No minimal sample size or dosing regimen was required for inclusion.
Number	Exclusion criteria
1	The report focused exclusively on other topics, such as addiction or sensitivity.
2	No human data were included.
3	The human 118A > G variant was not included, or no data were reported for this variant.
4	The genotype distribution of the control population was not in accordance with the Hardy-Weinberg equilibrium (HWE).
5	Reviews and duplicated publications.