Aspirin, warfarin combo boosts bleeding risk

Illustration of a vial of blood on a blue background.

March 26, 2019—It's always best to tell your doctor if you're thinking of taking a new medicine. That's especially true if you're already on another drug. One reason? Mixing drugs may increase your risk of side effects. A case in point: aspirin and the anticoagulant (blood thinner) drug warfarin, as a new study highlights.

The study found that people who take both aspirin and warfarin face a heightened risk of serious bleeding—without getting much benefit.

Clot prevention

Many people with atrial fibrillation (abnormal heartbeats) or venous thromboembolic disease (a condition that includes deep vein thrombosis and pulmonary embolism) are prescribed warfarin. The goal? To cut their risk of blood clots that can travel to the brain or the lungs and cause a stroke or a pulmonary embolism. And aspirin is used to lower the risk of conditions such as heart attacks, strokes and colorectal cancer.

But one risk of taking a blood thinner is serious stomach bleeding. Taking two blood thinners at once may raise that risk even more.

In this study, researchers looked at more than 6,500 people who visited anticoagulation clinics. (People on warfarin go to clinics to have regular blood tests to make sure their medicine is working properly.) More than one-third of these people were also taking aspirin without any clear reason, according to the research team.

Among their results:

  • 5.7 percent of those on both blood thinners had major bleeding problems. That's compared to 3.3 percent of those who just took warfarin.
  • Those on both warfarin and aspirin without a clear reason also had to go to the emergency room or stay in a hospital for bleeding more often than did those on just warfarin.
  • Despite these increased risks, those on both blood thinners did not seem to gain any benefits, such as fewer heart attacks, strokes or blood clots.

Good communication is key

For some people, taking aspirin and warfarin actually does make sense, the study authors noted. This may include people with peripheral artery disease or who've had a heart attack or certain heart procedures. But only a doctor can decide if taking both drugs is really necessary.

Again, one big takeaway from this study is the need to be open with your doctor. If warfarin is prescribed for you and you already take aspirin (or vice versa), make sure your doctor knows.

In fact, even though you can buy aspirin over the counter, you shouldn't start taking it every day without talking to your doctor first. While aspirin is generally safe, it isn't right for everyone.

The study appears in JAMA Internal Medicine.

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