Prescribing low doses of aspirin in an attempt to avoid heart disease or certain kinds of cancer has become common, but there are downsides that may outweigh benefits, especially for women, researchers say.

While a low-dose aspirin regimen in women under age 65 has been found to slightly lower risks of possible heart attacks, strokes and cancer of the colon, researchers have found it also can lead to an increased risk of significant gastrointestinal bleeding.

While low-dose aspirin, if prescribed by a doctor, can help prevent a repeat heart attack in someone who's already suffered one, its use in preventing a first-time heart attack -- what doctors term  "primary prevention" -- is a little more complicated, says cardiologist Dr. John Erwin of Scott & White Memorial Hospital in Texas.

"There's no question that aspirin can be a lifesaver for people who've already had a heart attack," says Erwin, who was not involved in the new study.

However, as a primary prevention it's unclear if any benefits outweigh risks of aspirin use, which can include stomach bleeding, ulcers, or even sometimes bleeding into the brain.

 The American Heart Association recommends that only people considered at "high risk" of a heart attack should consider taking aspirin as on ongoing regimen.

"It's been a huge conundrum for us over the years," Erwin says. "When it comes to primary prevention, there are relatively few patients who will get a big benefit. And there's always the risk of harm."

Reporting on their study in the online journal Heart, the researchers echo that concern, suggesting long-term low-dose aspirin for prevention "is ineffective or harmful in the majority of women with regard to the combined risk of CVD (cardiovascular disease,) cancer and major gastrointestinal bleeding."

For women older than 65, the researchers noted, there was also an increased risk of bleeding, but the benefits against heart disease and colon cancer were bigger than in younger women.

Nancy Cook, one of the researchers of the new study, says its findings suggest the risks outweigh benefits for most women under the age of 65.

"I probably wouldn't take aspirin unless I had a very high risk of either cardiovascular disease or colon cancer," says Cook of Brigham & Women's Hospital in Boston and Harvard Medical School.

What it comes down to, Erwin says, "is that people need to have a careful discussion of the potential benefits and risks [of aspirin] with a doctor they trust."