WASHINGTON -- A study of people with a hereditary disposition to colon cancer adds to the evidence that taking a daily aspirin lowers a person's risk for that disease, the third-most common cause of cancer in men and women.
Among a group of people with what is known as Lynch syndrome, the study found that those who took daily aspirin for two years were 60 percent less likely to develop cancer of the colon or rectum than those not taking the drug.
Many studies over the past two decades have suggested, but not proved, that taking aspirin helps protect against colorectal cancer. The evidence, however, hasn't been strong enough to persuade public-health experts to recommend the drug for that purpose.
An experiment answering the question definitively probably will not be done because it would cost too much and take too long. For millions of people taking aspirin to lower the risk of heart attack, the question is largely irrelevant. Whether this study will change physicians' advice to others is uncertain.
"I believe there is already strong evidence for aspirin use and reduced risk for colorectal cancer," said Eric Jacobs, an epidemiologist at the American Cancer Society. "The question is really about the overall balance of risks and benefits," something not addressed in the new study.
Aspirin causes one additional bleeding ulcer for every 1,000 people using it for a year. It also increases the risk of bleeding in the brain, especially in the elderly. In 2007, the U.S. Preventive Services Task Force, a federally appointed panel of experts, recommended against its use "to prevent colorectal cancer in individuals at average risk."
Asad Umar, an expert on gastrointestinal cancer prevention at the National Cancer Institute, said that for people with Lynch syndrome, daily aspirin use is worthwhile. However, he said, his personal belief is that "the data is not really there" to recommend its use for colon cancer prevention in most people.
Fifty percent to 70 percent of people with Lynch syndrome develop colorectal cancer. The disease occurs earlier and progresses more rapidly, making them a group for whom preventive measures can be more easily studied than the general population.
In the new study, published in The Lancet, researchers randomly assigned 861 people in 16 countries who had Lynch syndrome to take two aspirin tablets a day or identical-looking placebos. After slightly more than two years, there was no difference between the two groups in cancer incidence.
But after about five years, there was a difference. Eighteen people taking aspirin developed cancer compared with 30 taking a placebo. Not every participant stuck with the program: When only those who took aspirin for two years or more were considered, the reduction in cancer rate was 59 percent.
People with Lynch syndrome are at higher risk for some other cancers, such as uterine, pancreatic, and stomach cancer, and aspirin cut their risk for those, too.
A study in The Lancet early this year of 25,000 people in eight different clinical trials found that aspirin reduced cancer deaths by 21 percent. The trials were set up to look at aspirin's effect on preventing heart attacks, not cancer.
The effect on cancer of the esophagus, pancreas, brain, and lungs was seen in about five years, but the reduction in colon and several other cancers appeared later.
Numerous studies also have shown that the effect remains for at least a few years after a person stops taking aspirin.