Many earlier studies have suggested aspirin might be a way to prevent polyps and cancer, but the new analysis is the first to put the idea to a rigorous test.
Experts say that while aspirin's benefits appear to be small, it may still be a reasonable option for those at moderately high risk of this malignancy, the most common after lung cancer.
"It's clear aspirin will not be a magic bullet," said Dr. John Baron of Dartmouth Medical School, who directed the study. "You can't take an aspirin and do nothing else."
The experiment was intended to see if aspirin prevents a recurrence of polyps after the growths have been removed during routine colonoscopies. It found the 80-milligram baby aspirin size taken daily reduces this risk by 19 percent.
The dose is the same one already taken by millions of Americans to prevent heart attacks. The new work suggests they may be getting an additional benefit.
Until now, the strongest evidence of aspirin's cancer-preventing powers came from large population reviews that show regular aspirin users have only about half as much colon cancer as usual. However, those reports do not prove that aspirin — rather than some other lifestyle habit — is responsible, so the latest study was undertaken to see if people given aspirin solely to prevent polyps truly have fewer of them.
The effect it found is only about half as powerful as the earlier reports suggested it would be. Nevertheless, even this much reduction could have substantial impact, considering that colon cancer is the second-leading cancer killer, responsible for 48,000 deaths annually.
Federal health officials say they are close to recommending that daily aspirin be considered for people who have polyps removed, although they are likely to hold off until the outcome of a second aspirin study is known later this year.
"It's getting to the point where, for patients who are at moderately elevated risk of colon cancer, aspirin would be a reasonable option," said Dr. Ernest Hawk, chief of gastrointestinal research at the National Cancer Institute. "I will feel more comfortable with this when we have the results of the second trial."
Baron's study, sponsored by the cancer institute, was presented Sunday in San Francisco at a meeting of the American Association for Cancer Research.
The research was done on 1,121 otherwise healthy men and women in nine cities who had polyps removed during routine screening. They were randomly given aspirin or dummy pills.
During repeat screening three years later, the researchers found that 38 percent of those getting baby aspirin had new polyps, compared with 47 percent of people getting placeboes. However, little benefit was seen among those getting full-size aspirin. Their recurrence rate was 45 percent.
Aspirin interferes with blood clotting and can cause bleeding and digestive ulcers. "Aspirin does have some real risk, and that makes me pull back" from a stronger recommendation, Baron said.
Three large studies are under way to see if Vioxx and Celebrex, the widely prescribed arthritis pills, can prevent regrowth of polyps more safely and perhaps more powerfully. These drugs have action similar to aspirin but are less likely to cause ulcers.
Aspirin, Celebrex, ibuprofen and other nonsteroidal anti-inflammatory drugs all block production of prostaglandins, which cause inflammation and appear to fuel the growth of colon polyps. They may be involved in other tumors as well, including cancer of the prostate, intestines, breast, skin, lungs, bladder and tongue.
Hawk hopes the second aspirin study will settle the case for aspirin. It is being conducted by Dr. Robert Sandler of the University of North Carolina to check the drug's effect on colon cancer survivors.
A colonoscopy every 10 years is recommended after age 50. The screening typically is repeated after three years if polyps are found. A polyp can grow to a few millimeters in size in a year or two, but it typically takes 10 or 15 years for them to become cancerous.
By Daniel Q. Haney