Dr. Bryan A. Cotton, a trauma surgeon in Houston, had not heard much about the new anticlotting drug Pradaxa other than the commercials he had seen during Sunday football games.

Then people using Pradaxa started showing up in his emergency room. One man in his 70s fell at home and arrived at the hospital alert and talking. But he rapidly declined. “We pretty much threw the whole kitchen sink at him,” recalled Dr. Cotton, who works at Memorial Hermann-Texas Medical Center. “But he still bled to death on the table.”

Unlike warfarin, an older drug, there is no antidote to reverse the blood-thinning effects of Pradaxa.

“You feel helpless,” Dr. Cotton said. The drug has contributed to the bleeding deaths of at least eight patients at the hospital. “And that’s a very bad feeling for us.”

Pradaxa has become a blockbuster drug in its two years on the market, bringing in more than $1 billion in sales for its maker, the privately held German drug maker Boehringer Ingelheim.

But Pradaxa has been linked to more than 500 deaths in the United States, and a chorus of complaints has risen from doctors, victims’ families and others in the medical community, who worry that the approval process was not sufficiently rigorous because it allowed a potentially dangerous drug to be sold without an option for reversing its effects.