Three new oral blood-thinning drugs nearing approval by the U.S. Food and Drug Administration are more convenient than the standard drug Coumadin because they don't require monthly visits to adjust doses.

The promising drugs, though, 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, a Loyola University Health System study has found.

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

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

The lead author of the study is blood clot specialist Jeanine Walenga, a professor in the Cardiovascular Institute and the departments of thoracic and cardiovascular surgery and pathology at the Loyola University Chicago Stritch School of Medicine. Dr. Cafer Adiguzel, who completed a fellowship at Loyola, is the study co-author.

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

The drug, also known by its generic name of warfarin, must be monitored carefully.

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. Typically, patients must come in every month for a blood test to determine whether their doses need to be adjusted.

The three new drugs don't 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 of them from the FDA.

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 U.S. consume at least one of the drugs known to have some level of interaction with one of the new oral anticoagulants," the Loyola researchers wrote.

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 monitored carefully. 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.