The Food and Drug Administration has rejected Johnson and Johnson‘s blood thinner Xarelto as a treatment for heart disease for the second time. The drug is approved to prevent strokes in patients who have atrial fibrillation, a heart rhythm disorder, and to prevent blood clots in patients undergoing knee and hip replacement surgery.

There had initially been some hope that the drug would be approved based on a study, published in the New England Journal of Medicine, that showed that giving Xarelto on top of other drugs like statins and Plavix to patients hospitalized for chest pain or after a heart attack reduced the risk of heart attack or death from cardiovascular causes by 16%, but it also quadrupled the risk of bleeding. The FDA apparently decided that the bleeding risk outweighed the prevention of heart attacks. Another trial in similar patients would probably be too expensive for J and J and its partner, Bayer, to contemplate.

One interesting fact: Xarelto was the only one of a group of similar drugs, called Factor X inhibitors, to have a positive result in preventing heart attacks. It’s not clear why Xarelto might have been different from Eliquis, the competitor from PFizer and Bristol-Myers Squibb, which failed to show a benefit in such heart patients.

There is some disagreement among analysts about how big a product Xarelto will be. Lawrence Biegelsen at Wells Fargo, who is bullish on J&J, forecasts 2013 Xarelto sales of $425 million. Catherine Arnold of Credit Suisse forecasts 2013 sales of $383 million.

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Matthew Herper, Forbes Staff