Robert S. Sandler, Dale P. Sandler, George W. Comstock, Knud J. Helsing, David L. Shore; Cigarette Smoking and the Risk of Colorectal Cancer in Women. J Natl Cancer Inst 1988; 80 (16): 1329-1333. doi: 10.1093/jnci/80.16.1329
Colorectal cancer incidence rates for smokers, nonsmokers living with smokers (i.e., passive smokers), and non-smokers in smoke-free households were compared in a 12-year prospective study of 25, 369 women who participated in a private census conducted in Washington County, MD, in 1963. Women who smoked had a decreased relative risk of colorectal cancer compared with the risk for nonsmokers (age-adjusted relative risk, 0.76; 95% confidence interval, 0.52–1.10). The risk for passive smokers was similar to that for smokers. The relative risks were significantly reduced for older women; relative risks were 0.42 for smokers and 0.66 for passive smokers over age 65. The data suggest that older women who smoke have a lower risk of colorectal cancer than non-smokers. The effect may be mediated by an antiestrogenic effect of smoking.