BACKGROUND: Diazepam is one of the most commonly prescribed tranquilizers for the therapy of alcohol withdrawal syndrome (AWS). Despite its popularity, there is currently no precise information on the effect of genetic polymorphisms on the efficacy and safety of diazepam therapy. OBJECTIVE: The objective of our study was to study the effect of CYP3A isoenzymes activity on the efficacy and safety of diazepam in patients with AWS. METHODS: The study was conducted on 30 Russian male patients suffering from the AWS who received diazepam in injections at a dosage of 30.0 mg / day for 5 days. The efficacy and safety assessment was performed using psychometric scales and scales for assessing the severity of adverse drug reactions. RESULTS: Based on the results of the study, we revealed the differences in the efficacy of therapy in patients with different () genotypes: () -9.0 [-13.0; -5.0], () -13.5 [-15.0; -10.0], p = 0.014. The scores on the UKU scale, which was used to evaluate the safety of therapy, were also different: () 7.5 [6.0; 11.0], () 11.0 [8.0; 12.0], p = 0.003. CONCLUSION: Possible relationship between the CYP3A activity, evaluated by the content of the urinary endogenous substrate of the given isoenzyme and its metabolite, the 6-beta-hydroxy cortisol (6-β-HC) / cortisol ratio, and the efficacy of diazepam was demonstrated. This possible relationship was also supported by the genotyping results. This should be taken into consideration when prescribing this drug to such patients in order to reduce the risk of pharmacoresistance.