Summary: A detailed analysis of aspirin use in women shows that the drug may reduce the risk of colon cancer, Harvard Medical School researchers report in the Journal of the American Medical Association. But women must take high doses for at least 10 years in order to see this benefit, and they risk potentially serious side effects.

Why it's important: Colon cancer is the second leading cause of cancer death in the US and the third most common cancer among men and women. Although screening tests can catch colon cancer early or find colon growths (polyps) before they become cancer, many adults who should get screened don't. Doctors have been studying whether aspirin or similar drugs can prevent colon cancer in hopes of finding another way to reduce rates of this disease.

What's already known: Many other studies, including a large American Cancer Society study published in 1991, have linked regular aspirin use to a lower rate of colon polyps and colon cancers. Similar drugs called non-steroidal anti-inflammatory drugs, or NSAIDs, also appear to have this benefit. But it's not clear what dose is needed and how long one of these drugs must be taken to have this anti-cancer effect. It's also not clear how big a problem side effects might be: aspirin and NSAIDs can cause potentially serious gastrointestinal bleeding.

How this study was done: The researchers collected data on nearly 83,000 women taking part in the Nurses' Health Study. Between 1980 and 2000, the women gave regular updates on their health, including whether they were taking aspirin or NSAIDs, how much they took, and how often they took it. Most of the women were taking the drugs for pain relief.

What was found: Over the 20 years of the study, 962 women developed colon cancer. Women who used aspirin regularly (taking 2 or more standard -- 325 mg -- tablets per week) had a lower risk of colon cancer than women who did not use aspirin, but the difference didn't become significant until the women had been using aspirin for 10 years or more. The more aspirin a woman used, the lower her risk. Women who took more than 14 standard tablets per week for longer than 10 years had about half the risk of women who did not use aspirin. The researchers found similar results when they looked at use of other NSAIDs (such as ibuprofen). Regular use of acetaminophen, another type of pain reliever that works in a different way, did not have an effect on colon cancer.

These findings are in line with recently published results from the Women's Health Study, the researchers say. In that study, low doses of aspirin (100 mg every other day) did not have an effect on colon cancer. In this analysis, similarly low doses also had no effect; only high doses worked.

But the side effects of such high doses are a concern. More than 1,600 women developed gastrointestinal bleeding that required a blood transfusion or hospitalization. This problem was most common in the women who took more than 14 aspirin or NSAID tablets per week.

The bottom line: This analysis gives the most detailed picture so far of the relationship between aspirin use and colon cancer prevention -- at least in women. It shows that long-term use of standard aspirin doses are needed to have an impact on this disease -- using baby aspirin for a few years probably won't have a protective effect. But an ACS expert says it's too soon to recommend taking aspirin for colon cancer prevention.

"Use of aspirin at any dose to prevent cancer is not recommended by the American Cancer Society (or by any other group) because of the potential for serious side effects, particularly gastrointestinal bleeding," said Eric J. Jacobs, PhD, a senior epidemiologist at ACS. Jacobs recently co-wrote an editorial about the Women's Health Study suggesting higher aspirin doses might be needed to affect colon cancer risk.

"The American Cancer Society does strongly recommend that all men and women get a screening test for colorectal cancer beginning at age 50," he added. "Such screening has been proven to reduce the risk of dying from colorectal cancer and can even prevent cancer because precancerous polyps can be removed before they turn into cancer."

Citation: "Long-term Use of Aspirin and Nonsteroidal Anti-inflammatory Drugs and Risk of Colon Cancer." Published in the Journal of the American Medical Association (Vol. 294, No. 8: 914-923). First author: Andrew T. Chan, MPH, of Massachusetts General Hospital and Harvard Medical School, Boston.