Long-term tobacco smoking and colorectal cancer in a prospective cohort study.

Tobacco smoking has consistently been associated with colorectal adenomas, precursors of cancer, but the association with colorectal cancer itself has not been consistent. If colorectal cancer emerges only after a 35-year induction period, an association would unlikely be detected in studies where exposure assessment is of shorter duration. Most previous studies do not examine smoking of such duration and therefore do not account for the hypothesized 35-year induction period. By using the Cox proportional hazards models to estimate relative risks, we studied the association of long-term smoking and colorectal cancer risk in a population-based prospective cohort of 17,118 Swedish twins with up to 30 years of follow-up and information on smoking habits prior to baseline exposure assessment. Long-term heavy smoking was associated with a statistically significant 3-fold increased risk of colorectal cancer compared with never smoking (relative risk 3.1, 95% CI 1.4- 7.1). Examining colorectal cancer sub-sites separately, a non-significant 60% increased risk of colon cancer was observed only for heavy smokers and a statistically significant 5-fold increased risk was observed for rectal cancer. Our data lend some support to the hypothesis that heavy long-term cigarette smoking is associated with increased risk of colorectal cancer. Further elucidation of this association would be valuable from both etiologic and public health perspectives.