The mechanism responsible for the inter-patient variability response in opiate dependent individuals to opiate agonist (methadone) is unclear. We hypothesised that Polymorphisms at cytochrome enzymes and opiate receptor (OPRM1) may play important role in determining the clinical response to methadone. In this study, we aimed to investigate the types and frequencies of polymorphisms at the CYP 2B6, CYP 3A4 and OPRM1 gene loci among MMT patients in Malaysia. This knowledge may ultimately allow the development of personalised medications based on the genotype of each patient in the future. Personalised methadone therapy may be used as an option of treatment in opiate dependent individuals.