A blood-thinning medication used by people with a type of irregular heartbeat should not be taken with certain cholesterol-lowering drugs, a study suggests.

The blood thinner, Pradaxa (known generically as dabigatran), is often prescribed for patients with an arrhythmia called atrial fibrillation. It helps prevent blood clots and strokes, which are a particular problem with atrial fibrillation, or A-fib.

But people who take dabigatran along with the cholesterol-lowering statin drugs simvastatin or lovastatin have a higher risk of major bleeding compared to patients who take other statins, researchers say.

"The study was conducted because simvastatin and lovastatin would be expected to increase the amount of (dabigatran) that is absorbed and thereby increase the risk of bleeding with this drug, a property not shared by the other statins," said lead author Tony Antoniou of St. Michael's Hospital and University of Toronto.

The researchers conducted two studies of Ontario residents age 66 and older who began taking dabigatran between 2012 and 2014. First they examined a set of patients who had experienced ischemic stroke, then patients who had major hemorrhage, each matched to four similar people without these conditions.

All participants in the study were using a statin drug in the two months prior to the study.

Among almost 46,000 people taking dabigatran, there were 397 cases of ischemic stroke and 1,117 cases of major hemorrhage. Using simvastatin or lovastatin was not tied to risk of stroke, but they were tied to about a 46 percent increased risk of major hemorrhage, as reported in CMAJ.

"The nature of our databases don't allow us to gauge the severity of bleeding," Antoniou told Reuters Health by email. "However, we tried to capture only major episodes of bleeding by identifying those instances that resulted in either a hospital admission or emergency department visit."

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Dabigatran's bleeding risk is highest for people over age 75 and with kidney problems.

"Lovastatin and simvastatin can block the activity of a protein that limits the amount of (dabigatran) that enters the body from the gut," Antoniou said. With less of the "bouncer" P-glycoprotein, more dabigatran enters the body, and bleeding risk increases, he said.

Other statins do not block P-glycoprotein and would not be expected to interact with dabigatran, he said.

"Because other statins would not be expected to interact with (dabigatran), preferential use of those drugs should be considered in patients treated with (dabigatran) to attain the benefits of statin therapy while minimizing the excess risk of bleeding associated with lovastatin or simvastatin," he said.

A-fib affects at least 2.7 million people in the U.S., according to the American Heart Association.