Original Article Tobacco Smoking and Colorectal Cancer Risk: An Evaluation Based on a Systematic Review of Epidemiologic Evidence among the Japanese Population

Background: It is unclear whether tobacco smoking is related to colorectal cancer risk in Japan. We evaluated the association among the Japanese population based on a systematic review of epidemiologic evidence. Methods:Originaldata were obtained from searches ofMEDLINE using PubMed,complemented with manual searches. The evaluation of associations was based on the strength of evidence and the magnitude of association, together with biological plausibility as previously evaluated by the International Agency for Research on Cancer. Results: A total of six cohort studies and 15 case–control studies were thus identified. A sub-stantial degree of heterogeneity was observed in the association between smoking and colon cancer; most case–control studies published before 1994 reported an inverse association, whereas studies conducted over the last decade did not find any significant association. Recent cohort studies have shown a non-significant 20–40 % increase in colon cancer risk asso-ciated with current smoking. Several recent case–control studies and some cohort studies have identified a weak to strong positive association between smoking and rectal cancer. Conclusion: We conclude that tobacco smoking possibly increases the risk of colorectal cance