Cannabis Smoking and Risk of Cancer: A Meta-Analysis of Observational Studies

Background: Cannabis (also called marijuana or marihuana) is 1 of the most widely used illicit substances in the world. While cigarette smoking is a well-known risk factor of many cancers, effects of cannabis smoking on risk of developing cancer have remained unclear. Aim: This study is conducted to evaluate the association between cannabis smoking and cancer risk. Methods: We searched PubMed, Embase and the bibliographies of relevant articles to locate additional publications in October 2017. Two evaluators independently reviewed and selected eligible studies based on predetermined selection criteria. Observational studies such as cross-sectional, case-control, and cohort studies reporting odd ratios (ORs) or relative risk (RRs) for association between cannabis smoking and any kind of cancer risk were included. Subgroup analysis also was performed by cancer type (lung, oral, testicular, head and neck and others) and by smoking duration ( 10 years). Results: We included 18 observatio...