An updated review of the epidemiological evidence that cigarette smoking increases risk of colorectal cancer.

Carcinogens from tobacco reach the colorectal mucosa through either the alimentary tract or the circulatory system and could possibly damage or alter expression of important cancer-related genes. Twenty-one of 22 studies found that long-term, heavy cigarette smokers have a 2-3-fold elevated risk of colorectal adenoma. Risk of large adenomas, immediate cancer precursors, was elevated in smokers in 12 of 12 studies. The studies of smoking and colorectal cancer risk conducted earlier in the twentieth century consistently did not show any association. However, 27 studies in various countries, including the vast majority of those that have been analyzed in the past several years, now show an association between tobacco use and colorectal cancer. In the United States, 15 of 16 studies conducted after 1970 in middle-age men and elderly men and, in the 1990s, in women demonstrate an association. This temporal pattern is consistent with an induction period of three to four decades between genotoxic exposure and the diagnosis of colorectal cancer and with men as a group having begun smoking several decades earlier than women. Overall, accumulating evidence, much within the past decade, strongly supports the addition of colorectal cancer to the list of tobacco-associated malignancies and the possibility that up to one in five colorectal cancers in the United States may be potentially attributable to tobacco use.