People taking blood-thinning medications often use over-the-counter (OTC) medicines with the potential to cause dangerous internal bleeding, a recent study suggests.

The study focused on 791 patients prescribed apixaban, one of several newer blood thinners known as NOACs (non-vitamin K antagonist oral anticoagulants) that are recommended to prevent stroke in people with atrial fibrillation, a heart rhythm disorder.

Almost all of these patients used over-the-counter medicines, and 33% of them took at least one nonprescription drug daily or most days of the week with the potential to cause dangerous side effects when combined with apixaban. And almost 7% of them regularly took two or more over-the-counter medicines that could be a dangerous mix with apixaban.

"New OTC products are constantly being adopted by patients," Dr. Derjung Tarn of the David Geffen School of Medicine at UCLA and colleagues write in the Journal of the American Geriatrics Society. "This study demonstrates that patients have limited knowledge about potential serious interactions between OTC products and apixaban."

In atrial fibrillation, electrical impulses in the upper chambers of the heart are chaotic, causing the heart muscle to quiver rather than contracting normally. As a result, blood doesn't flow smoothly through the heart. This can lead to the formation of clots that can then travel through the arteries to the brain.

NOACs are the drug of choice for stroke prevention in patients with atrial fibrillation, which occurs most frequently in older patients. Apixaban is one of the most frequently prescribed. Others include dabigatran, rivaroxaban, and edoxaban.

Unlike the older blood-thinner warfarin, which required regular blood tests to prevent side effects, most people prescribed apixaban or other NOACs are not followed in specialized anticoagulation clinics or monthly by health care professionals, the study team writes. As a result, they may not be aware of potential drug interactions.