Background: Addiction is a polygenic disorder caused by genetic and environmental factors. The opioid material can act as an epigenetic element, like DNA methylation. 
Objectives: The present study aimed to examine the effect of epigenetic drugs such as nicotine, morphine, methadone, and buprenorphine on the methylation of two CpG sites in promoter of Oprm1 gene in male Wistar rats.
Materials & Methods: In this case-control study, 48 male Wistar rats with Mean±SD (200±30) g were divided into 6 groups: These are five groups only The control (intact) group, nicotine (0.4 mg/kg, SC injection for 5 days), morphine (10 mg/kg on days 1-3 and 20 mg/kg on days 4-6 and 40 mg/kg, IP injection on days 7-9), methadone (0.5 mg/kg, IP injection for 15 days), buprenorphine (0.05 mg/kg, SC injection for 6 days) and finally saline for each respected group. After the treatment, genomic DNA was extracted from the whole blood of the rats. Then, the extracted DNA was treated with sodium bisulfate. To identify the selected methylated areas of Oprm1 promoter sites (CpG-107 and CpG+33), we used methylation-specific PCR (MSP).
Results: Our results showed no methylation in the two CpG sites of Oprm1 promoter in all of the treated groups. 
Conclusion: Addiction with nicotine, morphine, methadone, and buprenorphine in doses and duration used in this study was not associated with the methylation of the Oprm1 promoter sites in the male Wistar rats.