well as catechol-O-methyltransferase – COMT). A candidate gene methylation study of the cannabinoid receptor 1 (CNR1) promoter region reported significant differences in CB1 expression when comparing cannabis dependent smokers against non-smokers as well as cigarette smokers (Rotter et al., 2012). Candidate gene studies of opioid use report significant differences in methylation of the promoter region of the gene encoding the μ opioid receptor (OPRM1). However, the direction of results is not consistent across studies (Nielsen et al., 2010; Nielsen et al., 2009).
