Cannabis Smoking and Risk of Lung Cancer - A Systematic Review and Meta-Analysis

Background: Cannabis is the illicit psychoactive substance the most consumed in the world. Little is known about the association between the use of cannabis and the risk of lung cancer. Objective: The objective of this meta-analysis is to determine whether use of cannabis is a risk factor for lung cancer. Methods: We conducted a systematic review and meta-analyses of all languages articles using relevant computerised databases. MEDLINE (online PubMed), Web of knowledge, Embase, EBSCO CINAHL, ScienceDirect, Scopus, Cochrane Library, and Directory of Open Access Journals were searched to September 2014 for cohorts and case-control studies that assessed the risk of lung cancer associated with cannabis smoking.  The literature search was performed with a combination of medical subject headings terms, "cannabis" and "lung neoplasms". Data extraction: Two investigators independently analysed and extracted results from eligible studies. Our study's registration number on PROSPERO is  CRD42014008872 . Results: The search strategy identified 2476 citations. 13 studies were eligible for inclusion: 2 pooled analysis of 9 case-control studies, one case-control study and 3 cohorts. The cumulative analysis for all the studies under a fixed-effects model showed that cannabis smoking determined an increased risk of developing lung cancer in the future (relative risk 1.22, 95% confidence interval 0.999–1.5; p=0.051), with no evidence of heterogeneity across the studies (I2: 34%; p¼0.01). Conclusions: The use of cannabis with or without tobacco smoking is associated with an increased risk for lung cancer.