Aspirin wards off heart attacks and strokes by preventing artery-blocking blood clots. But it's not for everyone. For example, the benefit against heart attacks applies mainly to men 45 and older, and the benefit against strokes is seen mainly in women 55 and older. Moreover, aspirin can cause dangerous gastrointestinal bleeding, so it's not for anyone with a history of stomach ulcers or other bleeding problems. Still, research suggests that only 20 percent of the women and 14 percent of the men who should be taking it are. And our recent survey found that many people underestimate the effectiveness of aspirin in protecting the heart. Here's how to know if aspirin therapy makes sense for you.

Know your risk

People of any age who are at very high risk of a heart attack or stroke should almost always take aspirin to protect their heart. That includes those who have:

• Already had a heart attack, stroke, or ministroke (a transient ischemic attack)
• Angina (chest pain)
• Undergone coronary-bypass surgery or angioplasty.
• Diabetes

Men between 45 and 79 without that history can also benefit from aspirin therapy, but only if they have an increased risk of having a heart attack in the next 10 years based on such factors as blood pressure and cholesterol levels and smoking status. Women between 55 and 79 often also benefit, but only if they are at elevated stroke risk based on many of the same factors.

To help make that determination, talk with a doctor and use our heart attack and stroke risk calculator.

Regardless of gender, the therapy should be limited to those who are not at increased risk of gastrointestinal bleeding. For men and women 80 and older, there's still not enough evidence to know for certain whether aspirin helps or not, so talk with a doctor to see if it might make sense in your case.

Use aspirin safely

To maximize the benefits and minimize the risks, take these steps:

Research generally suggests that they're as effective as standard adult tablets (325 milligrams) and are less likely to harm the gut and cause other complications. The one exception is if you think you're having a heart attack. In that case, call 911, then chew and swallow a 325 mg tablet.

If you have a history of stomach bleeding but your doctor says you need low-dose aspirin anyway, ask about adding a stomach-protecting drug such as misoprostol (Cytotec and generic) or omeprazole (Prilosec, Prilosec OTC, and generic). If you still can't tolerate aspirin or your risk of gastrointestinal bleeding is too high, ask him or her about using a different blood-thinning medication.

Stopping abruptly eliminates the protective benefits of aspirin and might elevate risk beyond original levels by making the platelets in the blood more likely to form clots. If you do need to stop, possibly before surgery, for example, do so for as briefly as possible.

Medications such as ibuprofen (Advil and generic) or naproxen (Aleve and generic) are chemically related to aspirin, so taken together they multiply the risk of stomach problems. So if you need pain relief, try acetaminophen (Tylenol and generic) or creams such as Zostrix that contain the topical anesthetic capsaicin. Moreover, ibuprofen and possibly naproxen can undermine aspirin's blood-thinning effect. If you do take aspirin and ibuprofen or naproxen, take the aspirin at least four hours before. That will give it a chance to do its job without having to compete against the other medication.