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I was sitting down to watch the bowl games on New Year's Day when the phone rang. It was my dad. A year ago, he'd called to tell me he was being admitted to a hospital in Florida with a heart attack. Thankfully, this time it wasn't his heart. But once again, he was in the hospital. He'd noticed his bowel movements were black and he felt a little lightheaded: two hallmarks of a gastrointestinal bleed.

Why would my father, who is in his 80s but in very good health, have a GI bleed? Well, in a way it was related to his heart. After his heart attack, he had stents placed in two of his coronary arteries to improve blood flow. To prevent the stents from becoming blocked, his doctors prescribed daily aspirin.

Aspirin works in part by inhibiting the clotting effects of platelets - cells that make your blood sticky. When there's a nick in a blood vessel, or an irritation in your intestinal lining, platelets work with proteins in your blood to stop the bleeding.

Aspirin in your blood prevents your platelets from sticking to each other and to plaque inside your arteries, which can help keep your blood flowing the way it should. But if you have any internal bleeding, aspirin can be a problem. To make matters worse, aspirin can directly irritate your stomach, causing irritation and even ulcers.

Fortunately, my father was treated quickly. Doctors threaded an endoscope into his small intestine and spotted an ulcer, then skillfully placed clips on the blood vessel that had been bleeding. Problem solved!

So what does this mean for other people considering daily aspirin? Was it wrong for my father to be taking it, and what about people who haven't had a heart attack - should they take aspirin to try to prevent one?

These are controversial topics. The data strongly suggest that if you've had a heart attack or stroke due to the blockage in an artery, aspirin can markedly reduce your risk of having another one. Most doctors will recommend that people in this group take an aspirin every day. However, if you haven't had a heart attack or stroke, it isn't as clear.

A recent study of 300,000 Italians taking aspirin to prevent a first heart attack or stroke found the number of cardiovascular events prevented was about equal to the number of major bleeding episodes caused by the drug. Because of the bleeding risk, the American Heart Association only recommends aspirin for people at high risk of having a heart attack, not for everyone. The U.S. Preventive Services Task Force agrees and recommends aspirin for men ages 45 to 79 and women ages 55 to 79 only when the potential benefits outweigh the harm due to gastrointestinal bleeding.

So for my father, aspirin was the right way to go. He was just unlucky to have suffered a complication seen in less than 3 percent of people taking aspirin for prevention of heart disease. But before you automatically start taking aspirin, remember that although it is sold over the counter, it's a real drug, with real side effects. Make sure you discuss with your doctor whether the benefits outweigh the risks.

"Tell Me the Truth, Doctor" is a weekly column written by ABC News' chief health and medical editor Dr. Richard Besser. Look for Dr. Besser's book April 23!