Long-term Aspirin Use Reduces Colon Cancer Risk, Study Shows

BOSTON--Aspirin appears to reduce the risk of colorectal cancerin women, but only if taken regularly for 10 years or more, reportEdward Giovannucci, MD, and his colleagues from Harvard MedicalSchool.

Their findings, based on analysis of data from the Nurses' HealthStudy (a large cohort of women followed since 1976), suggest thatintervention trials may require long follow-up to show beneficialeffects of aspirin. A previous intervention trial using low-doseaspirin saw no reduction in colorectal cancers during the first6 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 cohortin three consecutive questionnaires (1980, 1982, and 1984), andcases of colorectal cancer occurring from 1984 through 1992 wereincluded (331 new cases during more than 500,000 person-yearsof follow-up).

"We found little reduction in the risk of colorectal cancerduring the first 9 years of regular aspirin use (relative risk,0.97), as compared with the risk among nonusers, but for 10 ormore years of use, the relative risk was 0.63," Dr. Giovannuccisaid in his New England Journal of Medicine report (333:609-614,1995).

The risk reduction was not significant among the 10- to 19-yearaspirin users but rose to clear significance among women who hadused aspirin regularly for at least 20 years (44% risk reduction).The full reduction in colorectal cancer risk occurred at a levelof four to six aspirin tablets a week, with no further benefitseen with increased doses.

Dr. Giovannucci noted that controlling for risk factors for colorectalcancer, including diet, family history, smoking, weight, physicalactivity, and alcohol consumption, did not alter the results.In addition, despite a slightly higher rate of endoscopy, womenwho took aspirin had fewer adenomas removed and fewer large adenomas.

The researchers suggest that risk reductions appear slowly overtime because aspirin may exert its preventive effects only atthe earliest stage of the disease (small adenomas).