Opioid use disorder (OUD) is a major public health problem affecting millions of people worldwide. Although OUD is a chronic and relapsing disorder, a variety of pharmacological and non-pharmacological interventions are available. Medication-assisted treatment of OUD generally relies on competition for opioid receptors against the addictive substance. The mechanisms of this competition are to block or inactivate the opioid receptor or activate the receptor with a substance that is intermittent or long acting. Methadone and buprenorphine are two United States Food and Drug Administration-approved medications that have long-term positive effects on the health of opioid-dependent individuals. Although clinical studies of drugs generally demonstrate efficacy in thousands of people and toxicity is excluded, it cannot be predicted whether the given drug will cause side effects in one of the patients at the treatment dose. Individual differences can be explained by many biological and environmental factors. Variations in genes encoding drug metabolism or cellular drug targets significantly explain the variability in drug response between individuals. Therefore, for the effects of candidate genes to be accepted and included in individual treatment protocols, it is important to repeat studies on individuals of different ethnic backgrounds and prove a similar effect.