Background: Cancer is the second leading cause of death after cardiovascular disease. In recent years it has been hypothesized that opiate use could be a risk factor for cancer. This study aimed to evaluate a possible association between opiate use and common cancers using ecological statistics from around the world.Methods: To investigate the association we used ordinary linear regression models. The log10-transformed age-standardized incidence rate (ASR) of cancers was used as dependent variables in the models. We adjusted for smoking, alcohol use per capita, human development index (HDI), and body mass index (BMI) as confounding variables. We extracted these variables from different data sources including the GLOBOCAN 2012, the United Nations Office on Drugs and Crime (UNODC) annual reports, World Health Organization database, the United Nations Development Program (UNDP) report 2012, and published literature. We estimated two separate models for each cancer, one for males and the other for both sexes.Findings: Opiate prevalence ranged from 0.01% to 2.65% and its median was 0.20%. In the multiple regression models for both sexes, opiate use was significantly associated with bladder (β = 0.59), kidney (β = 0.16), oral cavity (β = 0.27), esophagus (β = 0.33), larynx (β = 0.17) and other pharynx (β = 0.36) cancers. In the models based on the male data, the coefficient and the significances were approximately the same for the above cancers but larynx cancer was no longer significantly associated with opiate use.Conclusion: There was a significant association between opiate use and risk of cancers. We suggest that more studies should be conducted, especially in high-risk areas of the world.