OPRM1 and Anxiety: Linear-regression models show that anxiety and OUD were negatively related to OPRM1 DNA methylation levels (β = −0.178, p = 0.046 and β = −0.165 p < 0.001, respectively); furthermore, females had a lower OUD prevalence (β = −2.123, p = 0.006) but higher anxiety impact scores appeared (β = 1.869, p = 0.039).