Can an aspirin a day help ward off a heart attack or stroke? That depends.

Scientific evidence shows that taking an aspirin daily can help prevent a heart attack or stroke in some people, but not in everyone. It also can cause unwanted side effects.

According to Dr. Robert Temple, deputy director for clinical science at the Food and Drug Administration (FDA), one thing is certain: You should use daily aspirin therapy only after first talking to your health care professional, who can weigh the benefits and risks.

"Since the 1990s, clinical data have shown that in people who have experienced a heart attack, stroke or who have a disease of the blood vessels in the heart, a daily low dose of aspirin can help prevent a reoccurrence," Temple said. A dose ranges from the 80 milligrams (mg) in a low-dose tablet to the 325 mg in a regular strength tablet. This use is known as "secondary prevention."

However, after carefully examining scientific data from major studies, FDA has concluded that the data do not support the use of aspirin as a preventive medication by people who have not had a heart attack, stroke or cardiovascular problems, a use that is called "primary prevention." Risks such as dangerous bleeding into the brain or stomach are still present.

The bottom line is that people with previous problems should talk to a health care professional to get an informed opinion about dose and frequency, Temple said.

Read more from FDA.