Among CYP2D6 opioid users, the presence of CYP2D6 interacting drugs was associated with substantial differences in the prevalence of individuals having anxiety (14.21 vs. 6.49%), depressive disorders (56.85 vs. 10.28%), anti-epileptic drugs (27.38 vs. 9.40%), and GERD (27.90 vs. 9.85%) (p < 0.001). The gene discussed is CYP2D6; the disease is gastroesophageal reflux disease.