It is estimated that almost 150,000 Americans will be diagnosed with colorectal cancer in 2005, and over 50,000 will die from it. While some risk factors for colorectal cancer (e.g., age, family history, genetics) cannot be controlled, others can. For instance, studies suggest that diets high in fat and low in calcium, fiber, and folate may increase the risk of colorectal cancer, and people who smoke cigarettes are also at increased risk.

Recent studies have suggested that in people with a history of colorectal adenomas (benign, precancerous tumors) or cancer, regular use of aspirin can reduce the risk of developing a recurrent adenoma within 1-3 years. But these studies have been relatively short, and have not yet determined the dose and duration of aspirin needed to reduce colorectal cancer risk.

A new study in the August 24/31, 2005 issue of the Journal of the American Medical Association found that women who used aspirin regularly over a long term had significantly reduced risks of developing colorectal cancer. Non-aspirin non-steroidal anti-inflammatory drugs (NSAIDs)—such as ibuprofen, for example—had a similar effect. However, at the high doses needed to prevent colorectal cancer, the risk of gastrointestinal bleeding was significantly elevated.

About the Study

This study included 82,911 women who were participating in the Nurses’ Health Study, an ongoing study in which enrollees respond to health-related questionnaires mailed every two years. The women were ages 30-35 in 1976, when the Nurses’ Health Study began.

For this analysis, the researchers collected data from questionnaires completed between 1980 and 2000. In the questionnaires, the women indicated whether they were regular users (two or more tablets per week) of aspirin or other NSAIDs and, if so, the frequency, dose, and duration they used the medications.

The researchers kept track of which participants were diagnosed colorectal cancer or reported any major episodes of gastrointestinal bleeding that required hospitalization or blood transfusion.

The Findings

Over the 20-year study period, 962 (1.2%) of the women were diagnosed with colorectal cancer. Compared with women who were not regular users of aspirin, regular users (two 325-milligram tablets per week or more) were 23% less likely to develop colorectal cancer.

When the researchers looked more closely at the data, they found that significant colorectal cancer risk reduction was not seen until women had been regularly taking aspiring for more than 10 years.

The benefit also increased with dose. Women who took 2-5 aspirin tablets per week had a non-significant 11% reduction in colorectal cancer risk, those who took 6-14 tablets per week had a significant 22% reduction, and those who took more than 14 tablets per week had a significant 32% reduction. The benefit was the greatest in women who used more than 14 aspirin tablets per week for longer than 10 years, resulting in a 53% reduced risk of colorectal cancer.

The women who regularly used other non-aspirin NSAIDs saw a similar colorectal cancer risk reduction, which also increased with higher doses.

The risk of severe gastrointestinal bleeding increased significantly with higher doses of aspirin. For those women who didn’t take aspirin, the risk of major gastrointestinal bleeding in one year was 0.77 out of 1,000. This risk increased to 1.57 in those who took more than 14 aspirins per week. Another way of stating this is if 637 women took this high dose of aspirin for a year, one of them would be expected to develop major gastrointestinal bleeding. Similar risks were seen with NSAID use.

How Does This Affect You?

This study suggests that aspirin can significantly reduce the risk of colorectal cancer in women, especially when taking more than 14 tablets per week for more than 10 years. However, these large doses were also associated with a significantly increased risk of major gastrointestinal bleeding. In fact, the authors of the study estimated for every 1-2 cases of colorectal cancer prevented by high doses of aspirin, eight women would have major gastrointestinal bleeding events.

In addition to helping prevent cancer, aspirin has other benefits, including reducing the risk of heart attack and stroke. For this reason, many people are taking a baby aspirin (80 milligrams) every day, under the supervision of their physician.

While this study suggests that low doses of aspirin may not prevent colorectal cancer, higher doses are associated with side effects, including severe gastrointestinal bleeding. However, the validity of this result is limited by the biases and confounding factors associated with observational studies like this one. Randomized clinical trials would shed more light on the risks and benefits of higher doses of aspirin. In the meantime, such high doses do not appear to be a safe and effective way to lower your risk of colorectal cancer.