BACKGROUND: The prevalence and burden of substance and opium use have increased worldwide over the past decades. In light of rapid population changes in Tehran, we aimed to evaluate the prevalence of opium and other substance use among adult residents in Tehran, Iran. METHOD: From March 2016 to March 2019, we utilized data from 8 296 participants in the Tehran Cohort Study recruitment phase (TeCS). We calculated the age-sex-weighted prevalence of substance use and the geographic distribution of substance use in Tehran. We also used logistic regression analysis to determine possible determinants of opium use. RESULT: We analyzed data from 8 259 eligible participants with complete substance use data and the average age of participants was 53.7 ± 12.75 years. The prevalence of substance use was 5.6% (95% confidence interval [CI]: 4.6- 7.1%). Substance use was more common in males than females (Prevalence: 10.5% [95% CI: 8.6- 12.6%] vs. 0.5% [95% CI: 0.2- 1.2%], respectively). The age-sex weighted prevalence of substance use was 5.4% (95% CI: 4.6-7.1%). Moreover, opium was the most frequently used substance by 95.8% of substance users. Additionally, we found that male gender (Odds ratio [OR]: 12.1, P < 0.001), alcohol intake (OR: 1.3, P = 0.016), and smoking (OR: 8.5, P < 0.001) were independently associated with opium use. CONCLUSIONS: We found that the prevalence of substance use in Tehran was 5.6%, and opium was the most frequently used substance. In addition, male gender, lower levels of education, alcohol, and tobacco consumption are the main risk factors for substance use in Tehran. Healthcare providers and policymakers can utilize our results to implement preventive strategies to minimize substance use in Tehran.