Long-term use of aspirin may be a cheap, effective way of warding off colorectal cancer for people who are at high risk, but bleeding risks make it a bad idea for the average patient, researchers said on Thursday.

British researchers found that daily doses of aspirin may be worthwhile for patients at high risk of colon cancer, reducing the risk by more than 70 percent over 10 years.

Colorectal cancer is the second-leading cause of U.S. cancer deaths, killing about 52,000 people a year.

Several studies have shown aspirin may help prevent colorectal cancer. Daily aspirin use can also protect against heart attack or stroke for people at risk.

But it can cause gastrointestinal bleeding and it may worsen some strokes that involve bleeding in the brain.

“As always, it comes down to the balance of likely risk and benefit in individuals,” said Peter Rothwell of Radcliffe Infirmary in Oxford.

Rothwell and colleagues analyzed data from two large randomized clinical trials of aspirin done in the late 1970s and 1980s. They were looking especially at long-term results because it takes about 10 years for precancerous adenomas or polyps to develop into colon cancer.

What they found is that patients who took a daily 300 mg or greater dose of aspirin were 37 percent less likely to have colorectal cancer after five years and 74 percent less likely to have it after 10 to 15 years.

“If you have no evidence of any increase in risk of colorectal cancer, then the risks and benefits are probably similar in magnitude - so (it’s) probably not worth taking on the basis of cancer prevention alone,” said Rothwell, whose study was published in The Lancet medical journal.

“However, if your risk of colorectal cancer is increased - for example, by a strong family history or by having had a colonic polyp or cancer in the past - then the benefits are likely to outweigh the risks,” he said in an e-mail.

NOT RECOMMENDED

Dr. Andrew Chan of Massachusetts General Hospital, writing in a commentary accompanying the study, said Rothwell’s work does provide “convincing evidence that aspirin, at biologically relevant doses, can reduce the incidence of colorectal cancer.”

“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,” Chan wrote.

A similar analysis of long-term daily use of an adult dose of aspirin - 325 mg or more - found support for prevention of colorectal cancer as well.

That study, published on April 18 in the Journal of the National Cancer Institute, concluded that because of aspirin’s high toxicity in doses greater than 80 mg a day, the risks of further study were not warranted, especially given that screening and colonoscopy work well at preventing colon cancer.

Originally made from the bark of the willow tree, aspirin is one of nature’s oldest drugs. It was first synthesized more than a century ago and scientists continue to explore its potential benefits.

A 2004 study of daily use of adult aspirin over a lifetime resulted in about 1 in 15 people having an aspirin-related complication and about 1 in 556 people dying.