Federal health regulators delayed a decision on a blood thinner from for a second time Friday, raising concerns on Wall Street about the potential blockbuster medication.

The drug, called prasugrel, is considered crucial to Indianapolis-based Lilly, which faces a wave of patent expirations in the next few years.

Lilly said in a statement the Food and Drug Administration has still not completed its review of the drug, which was submitted in January. Agency scientists already delayed a decision on June, saying they needed more time.

"This is a very large, complex submission, and it should not be surprising that delays occur," Jennifer Stotka, a vice president with Lilly, said in a statement. The company said it would not speculate on the cause of the delay.

Lilly developed the drug with Japanese drugmaker Daiichi Sankyo Co. Prasugrel is designed to treat patients with acute heart problems such as heart attacks or unstable angina who are at risk of developing blood clots.

The drug's approval is key for Lilly's financial outlook, as the patent on its best-selling drug, the anti-psychotic Zyprexa, is due to expire in 2011. Zyprexa contributed more than a quarter of the company's $18.6 billion in sales last year.

Analysts said prasugrel could bring in anywhere from $600 million to $1 billion in revenue.

(nyse: UBS - news - people ) analyst Roopesh Patel said it was hard to draw conclusions from Lilly's statement because it simply said the FDA had not completed its review.

"Based on this, one can't necessarily conclude as to whether or not the final outcome would be positive or negative," Patel said.

He thinks Lilly will have to wait weeks or maybe months to hear more from the FDA.

"You're not talking about two or three years before the FDA comes back with an answer," he said.

A delay that long, or a request for more trials could be just as bad as a rejection.

Prasugrel would compete against the blockbuster blood thinner Plavix, which loses patent protection in 2011. Analysts say prasugrel needs time to establish itself in the market before facing generic versions of Plavix, which will sell for a fraction of that drug's original price.

Plavix, made by (nyse: SNY - news - people ) SA and (nyse: BMY - news - people ), was the second-best-selling prescription medicine in the world last year, with sales of $7.3 billion, according to pharmaceutical data provider (nyse: RX - news - people ).

Bristol-Myers executives have said they consider prasugrel a niche drug that targets only a small percentage of Plavix patients.

A head-to-head study released last year showed that fewer patients taking prasugrel developed blood clots in stents or suffered heart attacks, strokes or heart-related deaths when compared with patients taking Plavix.

However, the study also showed that major bleeding occurred in a higher percentage of patients taking prasugrel.

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