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
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. 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
"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,
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 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.