THURSDAY, Sept. 16 (HealthDay News) -- Taking low-dose aspirin daily may reduce the risk of colorectal cancer, new research finds.

Prior research has shown higher doses of aspirin reduces the risk of colon cancer. But this study is among the first to show that lower doses -- which have less risk of side effects such as gastrointestinal bleeding -- may also be effective, said Eric Jacobs, strategic director of pharmacoepidemiology for the American Cancer Society.

For this study, researchers in Scotland asked 2,279 people with colorectal cancer and 2,907 people without colorectal cancer to answer questions about diet and lifestyle choices could affect cancer risk.

Participants were asked how often they took low-dose aspirin (75 milligrams), as well as non-aspirin NSAIDs (non-steroidal anti-inflammatory drugs) such as naproxen (Aleve, Naprosyn and others) and ibuprofen (Advil, Motrin and others) in the year prior to their cancer diagnosis or recruitment in the study.

About 18.1 percent of those without cancer reported taking aspirin during that time, compared to 15.5 percent of those who had colorectal cancer.

Participants who reported taking low-dose aspirin regularly for a year or more were about 22 percent less likely to have colorectal cancer. The reduction was clinically significant among those who reported taking low-dose aspirin for more than five years, according to the study: They were 30 percent less likely to develop cancer than their peers who didn't take the painkillers.

Although the greatest risk reduction occurred in those consuming more than 525 milligrams of aspirin a week, the protective effect was present even for the lowest daily dose.

"The new case-control study from Scotland is one of the largest to date to have specifically examined low dose aspirin, and provides some evidence that using low-dose aspirin for five or more years may also reduce colorectal cancer risk," said Jacobs, who was not involved in the research. "However, previous studies of low-dose aspirin and colorectal cancer have had mixed results, and results of this study need to be confirmed."

The study also found that taking any NSAID regularly was associated with a reduced risk of colorectal cancer, compared to those who didn't use the painkillers.

The subjects and controls had been matched for age, gender and locale as well as smoking, alcohol and fiber intake, and the findings held true even after results were adjusted for weight, body-mass index and other factors, according to the study, which was published in the current issue of the journal Gut.

However, taking NSAIDs of any type before diagnosis did not increase survival time in bowel cancer patients or influence the risk of death from any cause, researchers noted.

Previous epidemiological studies suggest higher doses of aspirin (325 miligrams daily, or a standard adult-strength aspirin) and other NSAIDs reduce risk of colorectal cancer, said Dr. Andrew Chan, an assistant professor of medicine at Harvard Medical School and attending gastroenterologist as Massachusetts General Hospital.

While the new findings are promising, since many Americans are already taking aspirin for cardiovascular disease, more research is needed before saying with certainty that low-dose aspirin is also effective, Chan said.

Because the study is retrospective, for example, it relies on people's recollections of aspirin use, which may not be accurate. The researchers also noted it was unknown whether the subjects with colorectal cancer continued to take NSAIDs after diagnosis, "which may clearly limit the conclusions drawn from the survival data presented."

"What is very well-established in the literature, based on prior studies and now this study, is that aspirin is clearly effective in reducing the risk of colorectal cancer," Chan said. "What is less clear and still remains somewhat controversial is what is the minimum effective dose in terms of prevention."

Because aspirin can have potentially harmful side effects, organizations such as the American Cancer Society don't recommend taking aspirin to ward off colorectal cancer.

"Decisions about whether to take aspirin should be made together with a physician and should be based on balancing the heart disease prevention benefits against the risks of side effects," Jacobs said.

Worldwide, colorectal cancer ranks second as a cause of cancer deaths, killing nearly half a million people a year, according to background information in the study.

Jacobs also stressed the adults should continue to get screened for colorectal cancer. "Colorectal screening tests can result in the detection and removal of colorectal polyps before they ever turn into cancer. The American Cancer Society strongly recommends all men and women 50 or older get a colorectal cancer screening test."

More information

The American Cancer Society has more on colorectal cancer.