The US Food and Drug Administration (FDA) announced its plan to conduct a new safety assessment of the blood thinner Pradaxa (dabigatran). The drug was approved by the FDA in October 2010 as an alternative to warfarin (brand name Coumadin). Unlike warfarin, Pradaxa has no antidote to reverse its anti-clotting effects.

Pradaxa, which is manufactured by Boehringer Ingelheim, has been linked to more than 500 deaths in the United States alone. The drug was released in Europe in 2008. Despite the hundreds of deaths related to the drug’s usage in the US, this past June, Boehringer Ingelheim asked European regulators to approve wider use of Pradaxa.

Since its emergence on the US market, the FDA has received an unprecedented number of adverse event reports related to Pradaxa. The drug ranked number one on a list of most frequent drugs in direct reports to the FDA in 2012.

Recently, an FDA study investigating the risk of bleeding in atrial fibrillation patients taking Pradaxa was questioned as inadequate. A publication by the American Heart Association called the study potentially “unsuitable for informing the care of patients.”

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. One issue with the study, however, is that it only considered the occurrence of bleeding events and not their potential outcomes, which could be severe or even fatal in the case of Pradaxa because of its lack of antidote.

The study ultimately concluded that the rate of bleeding events was higher for warfarin than dabigatran. It did not take into account the available data on adverse events associated with the drugs. “An earlier version of the study was criticized by health care professionals who said the conclusions reached by the FDA were ‘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.

In January, the Institute for Safe Medication Practices reported that an analysis of bleeding cases related to anti-coagulants warfarin (Coumadin), dabigatran (Pradaxa), and rivaroxaban (Xarelto) found that bleeding incidents from dabigatran usage “were about 5 times more likely than warfarin to result in death.”

Due to persistent concerns about Pradaxa’s safety, the FDA will perform a new assessment of the drug. On December 30, the FDA posted a request for public comment on a proposed protocol for the assessment of dabigatran. A source told Forbes that “the major purpose of the study will be to identify groups of patients who are at high risk for major bleeding events with dabigatran.”

Boehringer Ingelheim responded to the FDA’s request for public comment, saying, “The fact that FDA requests and receives data on a particular product through Mini-Sentinel does not necessarily mean there is a safety issue with the product.”

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