New blood thinners can cause dangerous drug interactions, study finds

But the promising drugs also could be subject to dangerous interactions when taken alongside widely used prescription drugs, over-the-counter medicines such as aspirin and even herbal supplements such as St. John's Wort, according to a Loyola University Health System study.

"Many unknowns remain as to how the new anticoagulants will behave in the real world patient population," researchers conclude in a review article in the June issue of the International Journal of Clinical Practice.

While the new drugs offer "significant potential advantages," their lack of extensive clinical experience "should not be underestimated," researchers wrote.

Lead author is blood clot specialist Jeanine Walenga, PhD, a professor in the Cardiovascular Institute and departments of Thoracic and Cardiovascular Surgery and Pathology at Loyola University Chicago Stritch School of Medicine. Co-author is Cafer Adiguzel, MD, who completed a fellowship at Loyola.

Coumadin reduces the risk of life-threatening blood clots in patients who have chronic conditions such as irregular heart rhythms or have undergone recent surgeries such as hip and knee replacements.

Coumadin (generic name, warfarin) must be carefully monitored. If the dose is too high, a patient could experience excessive bruising and be at higher risk for brain hemorrhages. If the dose is too low, the drug would be ineffective in preventing life-threatening blood clots. Patients typically must come in every month for a blood test to determine whether the dose needs to be adjusted.

The three new drugs do not need to be monitored every month, according to their manufacturers. They are rivaroxaban (Xarelto®), dabigatran etexilate (Pradaxa®) and apixaban. Rivaroxaban and dabigatran etexilate have been approved in Europe and apixaban is under development. Manufacturers of all three drugs have asked for or are expected to seek approval from the U.S. Food and Drug Administration.

Interactions with other drugs can make the new blood-thinning drugs either ineffective or too effective. For example, St. John's wort, typically taken for depression, can make the drugs less effective, while aspirin can make them more potent and thereby increase the risk of bleeding.

One study found that one-third of older adults use aspirin. A second study found that 29 percent of adults ages 57 to 85 take at least five prescription drugs.

"A high proportion of adults in the United States consume at least one of the drugs known to have some level of interaction with one of the new oral anticoagulants," the Loyola researchers write.

So far, most of what's known about potential drug interactions comes from animal studies or clinical trials that included relatively healthy patients who were carefully monitored. Less is known about elderly patients who have multiple health problems and might miss doses or take incorrect doses.

"The impact of any drug interaction will only become known with increasing clinical experience of these new oral anticoagulants," researchers wrote.