Background: Drug use disorders are one of the major health problems in societies, which cause physical, psychological, and social damages to individuals. Socioeconomic status is often inversely associated with drug use disorders. The present study aimed at determining the effect of socioeconomic inequality on the prevalence of drug use disorders and identifying its determinants in Iran. Methods: Data of 7886 individuals aged 15 to 64 years were collected from Iran Mental Health Survey (IranMHS). Initially, the socioeconomic status of the participants was determined by principal component analysis. Later, socioeconomic inequality was measured using the concentration index, and the factors influencing the gap between the high and low socioeconomic groups were identified using the Oaxaca-Blinder Decomposition. Results: The concentration index for drug use disorders in Iran was-0. 29 (standard error= 0. 06). The results of decomposition technique revealed that 1. 14% and 2. 7% of the participants with high and low socioeconomic status were affected by drug use disorders, respectively. In addition, the gap between these 2 groups was found to be 1. 65%. Among the studied variables, occupation, marital status, and gender accounted for the highest contribution to inequality, respectively. Conclusion: There is inequality in the prevalence of drug use disorders in Iran; these disorders are more common in lower socioeconomic group. Based on the findings, it is suggested that improvement in the socioeconomic status of the households, especially for males, the divorced or widowed individuals, and the unemployed may lead to a reduction in inequality in drug use disorders.