Two studies published in Thursday’s New England Journal of Medicine (Vol. 348, No. 10; 883-890) report that daily aspirin use lowered the chance of developing colorectal polyps, a frequent precursor to colon cancer, in people who had previously been treated for either colorectal cancer or polyps. However, experts said it is still too early to recommend aspirin as a colon cancer prevention method because of the risk of serious side effects from the drug.

Colorectal cancer is the second leading cause of cancer-related deaths in the US. The ACS predicts that more than 57,000 people will die of this cancer in 2003. Yet, it is one of the most preventable cancers.

Approximately 80% of colorectal cancers develop from benign polyps. This is a growth that starts in the lining of the colon or rectum. Eventually it may become cancerous, but this process takes, on average, 10 years. If the polyp is spotted by a screening test such as sigmoidoscopy or colonoscopy, it can be removed and will never have a chance to turn into cancer. In both of these tests a doctor actually looks at the inside of the colon and rectum.

Now doctors have examined whether aspirin can prevent polyps in the far more common situation of people who have already had colorectal cancer or a previously treated polyp.

One study, from the Cancer and Leukemia Group B, studied 635 people. All had early stage colorectal cancer treated by surgery to remove the cancer. As part of their follow-up they underwent colonoscopy at the end of the study to look for new cancers or polyps.

During the study, half the people were also given a single adult aspirin tablet daily while the other half received only a placebo (pill containing no aspirin). At the end of the study, only 17% of the people on aspirin developed a new polyp compared with 27% who got the placebo.

The second study, called The Aspirin/Folate Prevention Study, looked at people who had only polyps, but not cancer. After their polyps were removed, one-third of the patients were given a daily, low-dose baby aspirin, one-third were given the adult dosage of aspirin, and one-third were given a placebo.

As in the first study, the aspirin group also had fewer polyps when a follow-up colonoscopy was performed three years later. But the results weren’t as clear-cut because most of the benefit was seen in the patients receiving the baby aspirin. Their dose was about one-fourth that of those receiving the adult aspirin. The researchers couldn’t explain this difference but thought it might be due to chance.

Do these results mean that everyone -- or at least those with colorectal cancer or polyps -- should take aspirin? Not according to Dr. Thomas Imperiale, a colorectal cancer expert from Indiana University School of Medicine. In an accompanying editorial he pointed out that the side effects from aspirin (bleeding, stomach problems) might not be worth it since screening procedures like colonoscopy could catch the polyps before they turn into cancer.

American Cancer Society experts agree. "These studies are important because they demonstrate experimentally that aspirin can inhibit the formation of colorectal polyps in patients who have had a previous cancer or polyp, and not only in people with Familial Adenomatous Polyposis," said Michael Thun, Vice President of Epidemiology & Surveillance Research at the American Cancer Society.

"However," Thun points out, "they do not by themselves provide sufficient evidence to recommend aspirin for cancer prevention." Aspirin causes serious gastrointestinal bleeding in some people. Studies have not yet determined for which patients the benefits of inhibiting colorectal polyps with aspirin outweigh the risks of bleeding complications.

Aspirin use reduced, but did not completely prevent, polyp formation in these studies. "Although aspirin may ultimately reduce the frequency of recommended colonoscopy or other screening procedures, it is unlikely to replace these procedures," said Thun.

Right now the ACS recommends colonoscopy every 10 years or sigmoidoscopy every five years, along with yearly tests for blood in the stools. Whether aspirin can lengthen these time intervals needs to be tested before any recommendation is made.