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

Abstract only 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 observational studies with 2 cohort studies and 16 case-controls studies, which involved a total of 13,646 cancer patients and 151,572 participants without cancer in final analysis. The random-effects meta-analysis of all 20 studies showed marginally statistically significant association between cannabis smoking and risk of lung cancer (OR = 1.76, 95% CI 1.00-3.08; I 2 = 82.0%). Subgroup analysis by type of cancer shows that cannabis smoking more than 10 years increased risk of testicular cancer (OR = 1.50; 95% CI, 1.02-2.09; I 2 = 00.0%). Conclusion: The current meta-analysis of observational studies found an overall significant increased risk of lung cancer and cannabis. Further, an increased risk of testicular cancer when duration of cannabis smoking exceeded 10 years also was found.