Relationships between Aspirin Dose and Colorectal Adenomas

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The summary below is from the full report titled "A Prospective Study of Aspirin Use and the Risk for Colorectal Adenoma." It is in the 3 February 2004 issue of Annals of Internal Medicine (volume 140, pages 157–166). The authors are A.T. Chan, E.L. Giovannucci, E.S. Schernhammer, G.A. Colditz, D.J. Hunter, W.C. Willett, and C.S. Fuchs.

What is the problem and what is known about it so far?

Colorectal cancer is cancer of the colon (large intestine) or rectum. Most colorectal cancer develops from small growths called polyps. Adenomas, which are the most common type of polyp, are the only type that can become cancer. Doctors use flexible tubular instruments to examine the colon (colonoscopy or sigmoidoscopy) and find polyps. To distinguish adenomas from other types of polyps, they remove or take pieces of them (biopsies) and send the tissue for special examination (histologic examination).

Several studies show that taking aspirin helps prevent adenomas and thus colorectal cancer. Some studies suggest that short-term aspirin use can prevent adenomas. Other studies show that only long-term aspirin use can prevent them. In addition, some studies suggest that low-dose aspirin use does not work, while others suggest that low doses of aspirin do prevent adenomas.

Why did the researchers do this particular study?

To study the doses and the duration of use of aspirin that prevent colorectal adenomas.

Who was studied?

27 077 female nurses in the United States who had colonoscopy or sigmoidoscopy between 1980 and 1998. The nurses were 34 to 77 years of age and had no known history of colon disease, including adenomas or cancer, when they started the study.

How was the study done?

Beginning in 1976, the researchers mailed questionnaires every 2 years to the nurses. They asked about medical history and risk factors for cancer and heart disease. From 1980 through 1998, the researchers added questions about aspirin use and colonoscopy or sigmoidoscopy. The researchers reviewed the medical records of women who reported having these procedures to find out which women had polyps and then looked at the biopsy results for the polyps. They did not know which women had reported taking aspirin when they looked at these results. The researchers then compared reported aspirin use before the colon procedure among women with and without adenomas.

What did the researchers find?

Higher doses of aspirin were associated with lower risks for adenomas. Women who reported taking more than 14 aspirin tablets weekly had the lowest risk, and women who reported never using or rarely using aspirin had the highest risk. After aspirin dose and other risk factors for adenomas were taken into account, using aspirin for several years was not associated with lower risk than was using aspirin for shorter periods.

What were the limitations of the study?

Women self-reported aspirin use. Some may not have correctly remembered how often they used aspirin. In addition, the researchers could not assess exact doses (milligrams) that the women used.

What are the implications of the study?

Higher aspirin doses are associated with lower risks for colorectal adenomas than are lower doses. However, higher doses increase risks for adverse effects, such as bleeding, and lower doses prevent heart disease. People should not take higher doses of aspirin to prevent colorectal adenomas until more studies clearly define the overall benefits and harms of these doses.