A recent study by the US Food and Drug Administration investigating the risk of bleeding in patients taking the new blood thinner Pradaxa, has been questioned as potentially “unsuitable for informing the care of patients,” according to a publication by the American Heart Association.

Pradaxa (dabigatran) was approved by the FDA in October of 2010 as an alternative to the existing anti-clotting drug warfarin. Both are anticoagulant drugs used to prevent or treat blood clots and reduce the risk of stroke, particularly in patients with atrial fibrillation.

Unlike warfarin, however, there is no antidote to reverse Pradaxa’s blood-thinning effects. The drug has been linked to more than 500 deaths in the United States. Since its emergence on the market, an unprecedented number of adverse events related to Pradaxa have been reported to the FDA. In 2012, the drug ranked number one on a list of most frequent drugs in direct reports to the FDA.

In the American Heart Association’s journal Circulation, Dr. Jerry Avorn, a Professor of Medicine at Harvard Medical School and Chief of the Division of Pharmacoepidemiology and Pharmacoeconomics at Brigham & Women’s Hospital, states that while the FDA used the Mini-Sentinel Pilot Project to assess the rate of gastrointestinal and intracranial bleeding in patients who took either dabigatran or warfarin, their study only considered the occurrence of bleeding events, not their potential outcomes, which could be far more serious in the case of Pradaxa due to its lack of an antidote.

Instead the FDA study determined that the rate of bleeding events was higher for warfarin than for dabigatran and discounted the data on adverse events associated with the drugs.

Avorn further notes that the FDA analysis “was not adjusted for age, sex, or any clinical differences between the patients taking the two drugs, even though age and gender are independent risk factors for bleeding with anticoagulant use and the kind of atrial fibrillation patients in whom physicians would start the new drug could well have been different from those in whom they chose to continue using warfarin.”

An earlier version of the same study was “immediately criticized by health care professionals as ‘problematic’ and the conclusions reached by the FDA were described as ‘tantamount to a guess,’” said Ned McWilliams, a shareholder with the Levin, Papantonio law firm who practices in the areas of pharmaceutical and mass tort litigation. “It appears the FDA did nothing more than ineffectively attempt to provide itself political cover for approving such an unsafe drug.”

The Institute for Safe Medication Practices (ISMP) reported in January of this year that an analysis of bleeding cases related to three anti-coagulant drugs – warfarin (Coumadin), dabigatran (Pradaxa), and rivaroxaban (Xarelto) – found that dabigatran bleeds “were about 5 times more likely than warfarin to result in death.” Their analysis found that higher odds for a fatal outcome when using warfarin persisted after adjusting for differences in patient age and gender.

Despite the disparity between the FDA’s findings and those of the ISMP, and despite the high incidence of adverse event reports and patient bleeding deaths caused by Pradaxa, the drug remains on the market as “safe.”

Alisha is a writer and researcher with Ring of Fire. Follow her on Twitter @childoftheearth.