Taking Aspirin in the long-term may help prevent colorectal cancer in people at high risk of the disease, but the bleeding risks of the drug are too high to recommend it to the average person, researchers say.

Colorectal cancer is the third most common in cancer in Canada, with about 20,000 new cases diagnosed last year, and an estimated 8,500 deaths annually across the country.

In Friday's issue of the medical journal The Lancet, Peter Rothwell of Radcliffe Infirmary in Oxford and his colleagues analyzed follow-up data from two large randomized clinical trials done in the late 1970s and early 1980s.

It takes at least 10 years for precancerous polyps to develop into colorectal cancer, which is why the researchers were interested in the long-term follow-up.  

Among people who took a daily 300 milligram dose of Aspirin — known chemically as acetylsalicylic acid or ASA — for five years, the incidence of colorectal incidence fell by 37 per cent overall. After 10 to 15 years, the incidence fell by 74 per cent.

"Use of 300 milligrams or more of Aspirin a day for about five years is effective in primary prevention of colorectal cancer, with a latency of about 10 years, which is consistent with findings from observational studies," the team concluded.

Screening focus for prevention

Since Aspirin can cause gastrointestinal bleeding and may worsen some strokes, the benefits may outweigh the risks for someone with a strong family history, polyps or cancer in the past.

For cancer prevention in the general population though, the risks may outweigh the benefits.

The findings offer "convincing evidence that Aspirin, at biologically relevant doses, can reduce the incidence of colorectal cancer," Dr. Andrew Chan of Massachusetts General Hospital said in a commentary accompanying the study.

"However, with the concerns about the potential risks of long-term Aspirin use and the availability of alternative prevention strategies, these findings are not sufficient to warrant a recommendation for the general population to use Aspirin for cancer prevention."

The effects of different doses of Aspirin for preventing colon cancer remains unclear. Cancer societies and governments do not recommend it for prevention in people at high risk.

The Canadian Cancer Society does recommend that men and women age 50 and over have a fecal occult blood test at least every two years to help identify polyps before they become cancerous. Colonoscopies that look for polyps are also recommended for people most at risk.

A study published last month in the Journal of the National Cancer Institute also suggested taking 325 milligrams or more of Aspirin daily for at least five years was linked with a 30 per cent lower risk of colorectal cancer.

Risk factors for colorectal cancer include age, heredity, smoking, alcohol, obesity, a diet high in red meat and low in fruits and vegetables and a lack of physical activity.