"Of all the chemical compounds being investigated for preventing colorectal cancer, aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) are near the top of the list, particularly for people at high risk," says Dr. Thun, a lead investigator of one of the first large-scale studies to document the aspirin-colorectal cancer link.

Researchers began that analysis, part of the ACS's Cancer Prevention Study II, by asking more than 600,000 adults in 1980 about their aspirin use. Six years later, when researchers evaluated the rates and causes of death for the group, they found death rates from cancers of the digestive tract, including colorectal cancer, were about 40 percent lower among people who took aspirin 16 or more times a month.

Another study involved more than 11,000 men and women in Sweden who took NSAIDs for rheumatoid arthritis, writes Bernard Levin, MD, in his book, "Colorectal Cancer." Dr. Levin points out that the NSAID group in the study had a 37 percent lower rate of colon cancer and a 28 percent lower rate of rectal cancer than the general population. In addition, a study of US nurses noted a 30 percent overall reduction in colorectal cancer and a 50 percent reduction in advanced cases among users of NSAIDs, according to the book. A similar report on 47,000 male health professionals found those who used aspirin at least twice a week had a lower risk for colorectal cancer.

"We know compounds in NSAIDs help inhibit this disease experimentally," Dr. Thun says. "Current research is focusing on how best to administer these chemicals to minimize the side effects of NSAIDs and to identify who should or should not take aspirin long-term. Until this is known, doctors cannot recommend taking an aspirin a day to reduce or prevent colorectal cancer."

NSAIDs relieve inflammation and are used to relieve pain from arthritis, headaches, lower back strain, menstrual cramps and certain injuries. Common NSAIDs include ibuprofen (brand names Advil and Motrin), piroxicam (brand name Feldene) for arthritis and sulindac (brand name Clinoril) for arthritis, gout and joint pain. Aspirin, the best known NSAID, is also used for its blood-thinning qualities by many people who are at high risk for heart disease. Research has found daily aspirin use can reduce the occurrence of blood clots that trigger heart attacks and strokes.

However, the side effects of NSAIDs can be serious, particularly as the dose of aspirin-like drugs increases. Aspirin can irritate the lining of the stomach so much that it causes indigestion or nausea, or even stomach bleeding or ulcers, or problems with blood clotting, liver or kidney damage or stroke.

NSAIDs work by blocking the activity of the COX enzyme to make protaglandins, substances that can cause pain and inflammation but also have beneficial effects such as protecting the stomach. Certain prostaglandins are involved in a number of activities associated with cancer, including cell growth and reproduction, decreasing the normal death of cells and stimulating the growth of blood vessels which tumors need to grow.

"Studies have found, for example, that concentrations of a certain type of prostaglandin (known as PGE2) are higher in adenomatous polyps and adenocarcinomas than in normal tissue," Dr. Levin writes. "They may also prevent polyps from progressing into cancerous tumors." Research suggests NSAIDs reduce the number of polyps that develop along the intestinal lining in certain conditions.

"A study is under way, for example, that may determine whether 325 milligrams of aspirin each day for four years prevents adenomatous polyps from developing in individuals who have had surgery for early-stage colorectal cancer," Dr. Levin writes. "Another study is hoping to establish whether very small doses of aspirin (80 milligrams; about one-fourth the amount contained in a standard tablet) achieve the same results as higher doses."

Dr. Levin recommends that you talk to your doctor first if you are considering taking aspirin as a preventive strategy for digestive tract cancer or heart attacks.