New study results strengthen the evidence that people who smoke cigarettes over a long period of time have an increased risk for developing colorectal cancer, even after adjusting for other risk factors.

“This provides one more reason not to smoke, or to quit as soon as possible,” said senior author Michael J. Thun, M.D., M.S., vice president emeritus, epidemiology and surveillance research at the American Cancer Society. “Colorectal cancer should be added to the list of cancers caused by smoking.”

Findings are published in the December issue of Cancer Epidemiology, Biomarkers & Prevention, a journal of the American Association for Cancer Research, as part of a special focus on tobacco.

Thun and colleagues tested the association between long-term cigarette smoking and colorectal cancer after adjusting for multiple other factors that are generally associated with risk, including screening. From 1992 through 2005 the researchers followed almost 185,000 participants aged 50 to 74 years old; participants described their behaviors and medical conditions.

Participants who smoked cigarettes for 40 or more years, or who did not quit before age 40, had a 30 percent to 50 percent increased risk of developing colon or rectal cancer during the follow-up, even in analyses that adjusted for 13 other potential risk factors, according to Thun. After 13 years of follow-up, the researchers identified 1,962 cases of invasive colorectal cancer.

While previous large studies conducted in long-term smokers showed similar results, Thun stated that this study is the first to control for screening and all of the suspected risk factors for colorectal cancer, such as alcohol consumption, physical inactivity and consumption of red or processed meat.

“These findings contributed to the evidence recently reviewed by the International Agency for Research on Cancer (IARC) in October of this year,” Thun said. “IARC upgraded the evidence that smoking causes colorectal cancer from ‘limited’ to ‘sufficient’.”

This IARC reclassification brings the number of cancer organ sites causally related to cigarette use to 17, which includes cancers of the oral cavity, pharynx, nasopharynx, nasal cavity and paranasal sinuses, larynx, lung, esophagus (both squamous cell and adenocarcinoma), stomach, colorectum, liver, pancreas, kidney (both renal cell and transitional cell carcinoma), urinary bladder and lower urinary tract, uterine, cervix, and myeloid leukemia.

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