According to the American Cancer Society, an estimated 147,500 new cases of colorectal cancer will be diagnosed, and approximately 57,000 people will die of the disease this year. Previous research has suggested that aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) may reduce the incidence of colorectal adenomas (a type of pre-cancerous polyps) and colorectal cancer deaths. Left undetermined was how much aspirin should be taken or for how long before the average American adult could reap the benefits of its protective effects against this disease.

To address this question, a group of researchers set out to examine the influence of dose and duration of aspirin therapy in the primary prevention of colorectal cancer. The results of their study were published in the February 3rd issue of the Annals of Internal Medicine. The study found that regular short-term aspirin use does appear to help reduce the risk of colorectal cancer.

About the study

The researchers examined data from 27,077 participants in the Nurses Health Study, a large ongoing study of US female nurses that began in 1976. Since 1980, the study participants have completed detailed health questionnaires, including patterns of aspirin use, every two years. All of the participants were between the ages of 34 and 77 when they joined the study and had no history of colorectal adenoma or cancer, inflammatory bowel disease, or familial polyposis (an inherited condition that primarily affects the colon and rectum). However, they had all undergone either a colonoscopy or a sigmoidoscopy between 1980 and 1998.

The findings

The researchers were interested in the women’s relative risk for developing colorectal cancer. Relative risk is a measure of the association between an exposure and a health outcome. In this case, the exposure is aspirin therapy and the health outcome is colorectal cancer. A relative risk of 1.0 would mean that aspirin had no affect on the risk of developing colorectal cancer. A relative risk of 2.0 would indicate that a woman taking aspirin therapy had twice the risk of developing colorectal cancer as a woman who was not taking aspirin, and a relative risk of less than one would mean that aspirin had a protective effect against colorectal cancer. For instance, a relative risk of 0.80 would mean that a woman’s risk of developing colorectal cancer while taking aspirin therapy would be reduced by as much as 20%.

As shown in the table below, in this study, the researchers found that women who took more than 14 tablets of aspirin therapy per week received a greater protective effect than women taking smaller doses of aspirin or no aspirin at all. These dose-response relationships were similar whether the women had been taking the aspirin for a short period of time or over many years. In fact, women who had taken aspirin for more years than some of their counterparts appeared to receive no additional benefit.

Aspirin Dosage

Relative risk*

.5 to 1.5 standard aspirin tablets per week


2 or more standard aspirin tablets per week


2 to 5 standard aspirin tablets per week


6 to 14 tablets per week


More than 14 tablets per week


*As compared to women who were not taking aspirin therapy

How does this affect you?

The researchers concluded that regular, short-term use of aspirin reduces the risk of colorectal cancer. However, the greatest protective effect of aspirin therapy is seen at doses that are substantially higher than those recommended for the prevention of cardiovascular disease. Because one of the potential adverse effects of long-term aspirin use is gastrointestinal bleeding, further research is needed before aspirin therapy can be recommended for the prevention of colorectal cancer in the general population.

While large studies like the Nurse’s Health Study can be very powerful, they suffer from certain biases that are avoidable in other types of research, like randomized, controlled trials. For example, if the participants in this study did not accurately recall the amount of aspirin they took in the previous two years (a distinct possibility), the results may be thrown off significantly.

So, as alluring as the idea is that a widely available and inexpensive drug may play an important role in the prevention of a serious chronic disease, the researchers do not recommend the widespread use of aspirin at any dose for cancer risk reduction. Even low-dose aspirin can have adverse effects in some people. And, although the benefits of aspirin therapy may outweigh its disadvantages in certain high risk individuals (like those with a history of colorectal cancer), regular colonoscopy with polyp removal is still considered the most effective method of preventing colorectal cancer.