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

The drug 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,” said Jennifer Stotka, a vice president with Lilly in a statement. The company said it would not speculate on the cause of the delay.

Indianapolis-based Lilly developed the drug with Japanese drugmaker Daiichi Sankyo Co. It is designed to treat patients with acute coronary syndromes 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 it faces looming patent expirations. 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 say prasugrel could bring in anywhere from $600 million to $1 billion in revenue.

UBS analyst Roopesh Patel said it was hard to draw conclusions from Lilly’s statement because it simply says 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. Plavix also loses patent protection in 2011, and analysts say prasugrel would need time to establish itself in the market before facing generic competition.

The blockbuster Plavix, made by Sanofi-Aventis SA and Bristol-Myers Squibb Co., was the second-best-selling prescription medicine in the world last year with sales of $7.3 billion, according to pharmaceutical data provider IMS Health.

Bristol-Myers executives have said they consider prasugrel a niche drug that targets only a small percentage of Plavix patients. Plavix will lose patent protection in 2011, possibly opening the market to generic competition.

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 those taking Plavix.

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

Shares of Eli Lilly fell $1.82, or 3.9 percent, Friday to $46.82 in after-hours trading.

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AP Business Writer Matthew Perrone in Washington contributed to this story.