Sales of Eli Lilly & Co.'s (LLY) and Daiichi Sankyo Co.'s (4568.TO) new anti-clotting drug Effient appear sluggish, a Wall Street analyst said Monday, but the drug maker's finance chief said Lilly is making progress convincing health plans to pay for it.

The latest data for U.S. prescription volumes "reinforce our view that Effient's U.S. launch will be muted," Leerink Swann analyst Seamus Fernandez wrote in a research note Monday.

Fernandez, citing data from health-information provider IMS Health, said there have been 1,823 new prescriptions written for Effient since shortly after its August launch, which puts it significantly behind the most successful new drug launches of the past seven years.

The analyst said it's possible demand was hampered by vacation season, and that the next four to six weeks may provide a clearer picture of demand for the drug.

Another factor is that Effient is supposed to be given to patients initially in the hospital, and IMS data don't reflect such usage. The IMS data should show whether patients continue to use the drug after they're discharged.

"There will be a delay in prescription data showing up in IMS, given that initial use occurs in the hospital setting," Lilly Chief Financial Officer Derica Rice told investors at a UBS conference in New York, which was available by Webcast.

Rice said the first six months of Effient's launch will be critical to getting the drug placed on hospitals' preferred-drug lists, or formularies. He said Lilly has made "positive" progress in efforts to persuade health insurers to pay for the drug, including Tricare, the government-funded health plan for the military.

Outside the U.S., Rice said several countries have agreed to reimburse for the drug, such as the U.K. and Argentina. France has recommended reimbursement, but assigned its "lowest rating" to the drug, Rice said, though he noted that three-fourths of new products get such a rating. Further details about the French rating weren't immediately available.

Effient is competing with the blockbuster drug Plavix, which is co-marketed by Bristol-Myers Squibb Co. (BMY) and Sanofi-Aventis SA (SNY). Some analysts still see Effient exceeding $1 billion in annual sales eventually, but Wall Street enthusiasm for the drug has generally waned due to delayed regulatory approval and the bleeding risk associated with the drug.

Lilly shares recently rose 14 cents to $33.

-By Peter Loftus; Dow Jones Newswires; 215-656-8289; peter.loftus@dowjones.com