F.D.A. Delays Action on Eliquis From Pfizer and Bristol-Myers In a move that came as a surprise to researchers and analysts, the Food and Drug Administration has decided not to approve - for now - the anticlotting drug Eliquis, viewed as the next blockbuster for its makers, Bristol-Myers Squibb and Pfizer. The companies said on Monday that the agency had asked them for more information about managing and verifying data from a large study that compared Eliquis with warfarin, the current treatment standard for millions of people with a heart rhythm problem called atrial fibrillation. The request was the second major delay for the drug; in February, the F.D.A. asked for more time to consider the application. In a news release announcing the decision, Bristol-Myers and Pfizer said the agency had not asked for any new studies. "We don't believe that the information impacts the benefit-risk profile of the drug," said Laura Hortas, a Bristol-Myers spokeswoman. She said that the trial was complex, involving more than 18,000 people worldwide. The news, she said, "is disappointing, but it does not alter our confidence in the therapeutic profile of Eliquis." The decision surprised cardiologists and financial analysts, two groups that have closely followed the drug's development. Eliquis, which is also known by the scientific name apixaban, is considered by some to be the best of a new class of drugs aimed at replacing warfarin, which is inexpensive but requires careful monitoring to avoid dangerous side effects. "Warfarin has been the only player in this therapeutic arena for over five decades," said Dr. Sanjay Kaul, a cardiologist at Cedars-Sinai Medical Center in Los Angeles and a member of the F.D.A."s advisory panel for cardiovascular drugs. So the development of these new agents has been welcome news not only for physicians but also for patients. Two competitors are Pradaxa, from Boehringer Ingelheim, which was approved in 2010, and Xarelto, from Bayer and Johnson & Johnson, which was approved in 2011. When the results of the Eliquis study were made public last year, the drug was shown to prevent more strokes, with less risk of bleeding, than warfarin. "It hit on all the major positives - everything you could ask for," said Dr. Eric J. Topol, a cardiologist at the Scripps Clinic and a professor of genomics at the Scripps Research Institute. Of the most recent delay, he said: "There's something going on there that we don't know about, apparently. It's peculiar. The news also surprised C. Anthony Butler, a pharmaceutical analyst at Barclays, who described the development as "highly unexpected" in a note to investors on Monday. In the note, he said analysts had assumed that the drug would be approved and had estimated that it would bring in $100 million to $150 million in sales this year. He estimated that the drug would account for 20 percent of Bristol-Myers's profits by 2020. The impact on Pfizer, which takes in half of the worldwide profits, would be less because it is a bigger company, he said. Even if the F.D.A."s decision was unexpected, it may not have much impact on the drug's ultimate approval or sales, said Seamus Fernandez, an analyst for Leerink Swann. "It would certainly suggest that there's no real fundamental data integrity problem," he said. He estimated that the drug would bring in $2 billion in United States sales by 2016 and $3.2 billion by 2018. Ms. Hortas, the Bristol-Myers spokeswoman, said the agency had up to six months to issue a decision after the companies responded to the request. She said they were already working to provide the information.