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Patients who take the anticoagulant Pradaxa to reduce their risk of stroke now have another reason to discuss this choice with their doctors. The FDA approved Pradaxa in October 2010 for use in those with non-valvular atrial fibrillation, but since then, the drug has been linked to serious complications, including excessive bleeding that can lead to death.

Now, a new study from North Carolina suggests that there may be other risks with Pradaxa—mainly, that the medication can increase risk of viral medications, including the flu and myocarditis.

Pradaxa Convenient, but Lacks Antidote

Though Pradaxa was initially marketed as being more effective and convenient than the other main anticoagulant on the market—warfarin—as the number of prescriptions went up in 2011 and 2012, so too did the complaints. In November 2012, a study published in the Archives of Internal Medicine reported that patients taking Pradaxa after an acute coronary syndrome were three times as likely to suffer a bleeding event.

What’s most concerning about excessive bleeding associated with Pradaxa is that to date, there exists no readily available antidote. Patients taken warfarin who experience a fall or other injury that creates excessive bleeding can be treated with injections of vitamin K, which encourages blood clotting. There is no such treatment for patients on Pradaxa, making a bleeding event especially dangerous and even deadly.

A New Concern

Elderly patients and those with weakened immune systems may now have new reason to be concerned about Pradaxa. In an animal study, researchers from North Carolina and Berlin, Germany treated normal, healthy mice with Pradaxa, then studied their response to viral infections. They wanted to study how the body responds to viruses on a cellular level—they didn’t expect to discover the results that they did.

The results showed that when the mice taking Pradaxa were exposed to a virus that causes myocarditis—inflammation of the heart muscle that untreated, can lead to heart failure—the virus displayed an increase buildup in the heart and increased impairment of heart function. In other words, the Pradaxa seemed to increase the risk and severity of the infection.

Pradaxa May Inhibit the Immune System

As to why Pradaxa may have this effect on the body during viral exposure, researchers pointed to thrombin. An enzyme in the blood that causes clotting, thrombin is inhibited by Pradaxa. Researchers have recently discovered that thrombin also plays a large role in triggering the immune response against invading viruses.

It seems that when Pradaxa inhibits thrombin, it also inhibits the body’s ability to respond to viral infections. "Our findings show that blocking thrombin reduce the innate immune response to viral infection," said Nigel Mackman, Ph.D. and senior author of the study.

The researchers stated they are now working on a new study to find out if warfarin may create the same effects, or if these are specific to Pradaxa. Patients who are at risk of serious health complications from infections should talk to their doctors about their options.

Meanwhile, manufacturer Boehringer Ingelheim faces an increasing number of lawsuits filed around the country, with federal cases consolidated in the Southern District of Illinois.

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