More than one-third of people at risk for stroke are given the wrong medications, according to a recent study.

Researchers led by University of California heart specialist Jonathan Hsu, M.D., analyzed data from more than 500,000 people with atrial fibrillation, a heart rhythm issue that can increase the risk of stroke up to seven times, compared to people without the condition.

Approximately 40% of the patients included in Hsu’s study received aspirin instead of a prescription for anticoagulants, which can decrease their likelihood of stroke.

The possible side effects associated with anticoagulants may be to blame for their limited use in patients with atrial fibrillation.

“By prescribing aspirin, we may be fooling ourselves that the patient may be protected from stroke when this is not the case; blood thinners have been shown to be clearly more effective than aspirin,” Hsu told Reuters.

The results of the study, published in June in the Journal of the American College of Cardiology, found people prescribed aspirin to be younger, slightly less overweight and more likely to have comorbidities than those who were prescribed anticoagulants.