Is low-dose aspirin safe for you?

If you take daily aspirin, make sure you know why and understand the small but real bleeding risk.

Are you taking an aspirin a day to keep the cardiologist away? If so, you are among millions of Americans taking a daily dose of the cheap, widely available anti-inflammatory drug for "primary prevention" of cardiovascular disease. Primary prevention means you don't have cardiovascular disease, and hope that aspirin will help prevent it.

But there is a price for prevention. Aspirin suppresses clotting, which is the villain behind heart attacks and "ischemic" strokes (caused by a blocked artery in the brain). But aspirin also ups the risk for bleeding in the upper gastrointestinal (GI) region and brain. The critical question is whether your risk of cardiovascular disease outweighs the risk of bleeding. Right now, the answer is not simple.

"There is controversy about exactly who in primary prevention should get aspirin," says Dr. Deepak Bhatt, a professor of medicine at Harvard Medical School and chief of cardiology at the VA Boston Healthcare System. "It's important to have a discussion with your doctor, weigh the risks and benefits, and factor in the uncertainty to decide if you should be on aspirin."

Balancing benefits and risks

Taking low-dose aspirin for "secondary prevention" is not controversial. Secondary prevention is for people who already have had a heart attack, certain kinds of strokes, or other diagnosed cardiovascular disease that puts them at high risk of additional problems.

"If somebody already has evidence of cardiovascular disease, there's no question they should be on an aspirin unless they have some major bleeding issues or an allergy that prevents them from taking aspirin," Dr. Bhatt says.

In a group of 10,000 such people, aspirin can prevent 250 cardiovascular events, like heart attacks, strokes, and sudden death. Meanwhile, 40 cases of serious bleeding will occur. The ratio of risk to benefit is roughly six people helped for every one harmed. That's little consolation if you're sent to the hospital with internal bleeding, but as a public health policy this risk equation is acceptable.

Aspirin for healthy people

When aspirin is used to prevent cardiovascular disease, the scales tip more toward harm. For every 10,000 people taking low-dose aspirin, seven people will be helped—mostly by preventing heart attacks—to every four harmed. These numbers are averages, so the risk faced by an individual depends on his or her particular characteristics. The chance that aspirin will help rises with additional risk factors, like older age, being overweight, smoking, and having high cholesterol. The risk of bleeding also rises with age—but then so does the risk of heart attacks and strokes, and the potential benefit of taking aspirin.

A study in the June 6, 2012, Journal of the American Medical Association stoked the ongoing debate about low-dose aspirin for primary prevention. Researchers examined the health records of nearly 400,000 people in the Italian National Health Service. Twenty out of every 10,000 people experienced a major bleed—five times higher than the bleeding rate seen in previous clinical trials. Is this bad news for people taking aspirin?

Maybe—maybe not. Dr. Bhatt says that comparing the Italian results to previous clinical trials isn't as simple as it seems. "The Italian study examined real-world patients, including some who were probably at much higher baseline risk of bleeding than the patients in the trials. So, that might explain why the bleeding was higher," Dr. Bhatt says. "Also, what is missing is the degree of benefit of aspirin. Maybe that was higher too, but we don't know."

Because millions of Americans are now taking low-dose aspirin, even a small increase in the risk of major bleeding could affect a lot of people. But fundamentally, the Italian study told us what we already knew: "The balance between risk and benefit of aspirin for primary prevention is very narrow," Dr. Bhatt says, "and in many people the bleeding risk may outweigh the potential benefits."

What should you do?

For the time being, the science remains uncertain and experts don't agree on who should take aspirin to prevent a first heart attack or stroke. In Europe, for example, guidelines for cardiology do not recommend aspirin for primary prevention, citing an unfavorable ratio of risk to benefit. In the United States, the FDA has not approved any labeling for aspirin bottles regarding its use in preventing cardiovascular disease.

That may change in a few years, as results from new and better primary prevention trials are released. In the meantime, make sure you know where you stand on the scale of risk and benefit. And if you have prescribed yourself low-dose aspirin because of what you've read in the health press—seriously consider a chat with your doctor.