Opioids admnistration seems to be related to an increase of the degree of DNA methylation. In a study published in 2013, Doehring et al. rated the degree of DNA methylation in patients heroin addicts undergoing methadone substitution treatment. Patients treated with opioids have shown an overall increase in DNA methylation, in particular of μ-opioid receptor. It is interesting to note that the degree of DNA methylation correlates with the intensity of pain reported by subjects analyzed in the study.