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There is still a lot of uncertainty about the ability of aspirin to prevent bowel cancer. Most studies suggest some benefit, but there are conflicting data about the necessary dosage and the duration of use required before seeing a benefit. The evidence in support of other non-steroidal anti-inflammatory agents (such as Indocin, Motrin, Naprosyn) is even more uncertain. A new study from Harvard Medical School adds new information, but leaves a lot of questions. The investigators followed 82,911 women ages 30 to 55 at recruitment for more than 20 years, during which time 962 cases of bowel cancer occurred. The complicated findings were as follows:

1. Women who took one to two adult dose aspirin tablets a day had a 27 percent reduced risk of large bowel (colon) cancer. Those taking more than two aspirins a day showed a 36 percent reduced risk. Those who took two to five adult aspirin tablets a week had a 17 percent reduced risk that was not statistically significant.

2. Those taking more than two aspirins a day had a 19 percent reduced risk of cancer of the lowest part of the bowel (rectum), but it was not statistically significant.

3. The reduced risk of colon cancer was found only after ten years of use.

4. The reduced risk was found for early stage cancers, but not for more serious, more advanced cancers.

5. Non-steroidal anti-inflammatories other than aspirin produced similar reductions in colon cancer occurrence.

6. Intestinal bleeding requiring hospitalization and/or transfusion occurred about twice as frequently among those taking an aspirin a day or more.

Commentary: This study does not support the use of low dose aspirin for prevention of bowel cancer. And, it is hard to decide how to interpret the results with higher doses (one a day to more than two aspirin tablets a day) because of reduced risk for only the less advanced (less severe) bowel cancers. The modest benefit (about one-third reduced risk) of less severe colon cancer was balanced by increased risk of aspirin-induced bleeding. It is also not clear whether the investigators fully adjusted for repeated colonoscopy examinations with polyp removal that would reduce risk markedly.

Is there a bottom line? The investigators urge more studies. That about summarizes the current situation. At present, aspirin (or other non-steroidal anti-inflammatories) cannot be recommended for prevention of bowel cancer.

Chan, A.T., et al. Long-term use of aspirin and non-steroidal anti-inflammatory drugs and risk of colon cancer. Journal of the American Medical Association. Vol 294 (August 24) Pgs 914-923. 2005.

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