The U. S. Food and Drug Administration (FDA) on Friday approved Eli Lilly's blood thinner Effient, but the drug is required to carry the agency's sternest warning due to its bleeding risks.

The highly anticipated blood thinner is approved to reduce the risk of blood clots from forming in patients who undergo angioplasty, a common procedure to unblock a clogged coronary artery.

Effient tablets (prasugrel) will be the first real competitor to blood thinner Sanofi-Aventis and Bristol-Myers Squibb's Plavix, the world's second-best selling medication. The FDA's decision on Effient was delayed during a long 18 month review as agency officials weighed the drug's benefits versus its risks. Effient's approval was considered crucial for Lilly as patents protecting its four best-selling drugs expire by 2013.

A study of 13,608-patient was conducted by Lilly which compared their drug Effient to Plavix (clopidogrel), in patients with a threatened heart attack or an actual heart attack who were about to undergo angioplasty.

In the patients who received Effient the number of patients who had subsequent non-fatal heart attacks was reduced to 7.0% as compared to Plavix's 9.1%. The number of strokes and deaths were similar in the case of both the drugs but patients who had a history of stroke were found to be more likely to have another stroke when they took Effient. The drug also showed greater risks of significant, sometimes fatal bleeding in patients who took it.

The FDA said Effient should not be taken by patients with a history of bleeding, stroke or who are undergoing surgery. They added that the drug will carry a boxed warning, reserved for issues that can cause serious injury or death to alert physicians to the risks of "significant, sometimes fatal, bleeding."

Dr. John Jenkins, director of the Office of New Drugs, in the FDA's Center for Drug Evaluation and Research said, "Effient offers physicians an alternative treatment for preventing dangerous blood clots from forming and causing a heart attack or stroke during or after an angioplasty procedure. Physicians must carefully weigh the potential benefits and risks of Effient as they decide which patients should receive the drug."

According to Miller Tabak & Co. analyst Les Funtleyder the boxed safety warning could curb sales, but not to a large extent. "The FDA has been a lot more liberal with black box warnings than it was in the past, and in a way the black box has lost some of the meaning it had when it was rare," said Funtleyder. "But it still has the ability to somewhat limit sales."

Effient sales could reach an estimated $1 billion annually, compared with the $4.9 billion racked up by Plavix last year estimate Wall Street analysts.

Indianapolis-based Lilly developed Effient, in partnership with Tokyo-based Daiichi Sankyo Ltd. and the two companies will share revenue.

Daiichi President Takashi Shoda in a statement said, "After more than a decade of research and testing, we are proud to provide this new treatment option to patients with acute coronary syndrome."