BOSTON--Aspirin appears to reduce the risk of colorectal cancer in women, but only if taken regularly for 10 years or more, report Edward Giovannucci, MD, and his colleagues from Harvard Medical School.
Their findings, based on analysis of data from the Nurses' Health Study (a large cohort of women followed since 1976), suggest that intervention trials may require long follow-up to show beneficial effects of aspirin(Drug information on aspirin). A previous intervention trial using low-dose aspirin saw no reduction in colorectal cancers during the first 6 years of follow-up (J Natl Cancer Inst 85:1220-1224, 1993).
Reduction Seen at 10 Years
Patterns of aspirin use were determined in the Nurses' Study cohort in three consecutive questionnaires (1980, 1982, and 1984), and cases of colorectal cancer occurring from 1984 through 1992 were included (331 new cases during more than 500,000 person-years of follow-up).
"We found little reduction in the risk of colorectal cancer during the first 9 years of regular aspirin use (relative risk, 0.97), as compared with the risk among nonusers, but for 10 or more years of use, the relative risk was 0.63," Dr. Giovannucci said in his New England Journal of Medicine report (333:609-614, 1995).
The risk reduction was not significant among the 10- to 19-year aspirin users but rose to clear significance among women who had used aspirin regularly for at least 20 years (44% risk reduction). The full reduction in colorectal cancer risk occurred at a level of four to six aspirin tablets a week, with no further benefit seen with increased doses.
Dr. Giovannucci noted that controlling for risk factors for colorectal cancer, including diet, family history, smoking, weight, physical activity, and alcohol(Drug information on alcohol) consumption, did not alter the results. In addition, despite a slightly higher rate of endoscopy, women who took aspirin had fewer adenomas removed and fewer large adenomas.