Abbott Laboratories said that U.S. regulators have withheld approval of its experimental long-acting form of the painkiller Vicodin, leaving it unclear if and when the powerful narcotic might be approved.

But Abbott stuck to its earlier company earnings forecasts for 2008 and 2009 despite the uncertain fate of the pill, which it had expected to be approved in the fourth quarter and generate eventual peak annual sales of $500 million.

Like Vicodin, it contains the narcotic hydrocodone in combination with acetaminophen, an anti-inflammatory chemical that is the active ingredient in Tylenol. But it would be taken only once every 12 hours, compared with every 4 to 6 hours for Vicodin. The Abbott product is the first slow-release formulation of Vicodin, a generically available medicine for chronic pain that is one of the most prescribed drugs in the United States.

Company spokeswoman Melissa Brotz declined to disclose the contents of the Food and Drug Administration's so-called complete response letter, a document issued when the agency withholds approval until it receives information of some type. Companies have discretion to reveal the contents of such letters.

Brotz declined to say whether the agency was seeking more clinical trial data or new clinical trials, or when the drug might be approved. She said Abbott would discuss the letter with the FDA and provide an update when appropriate. The company reaffirmed that it still expects full-year 2008 earnings of $3.31 to $3.33 per share, which would reflect growth of up to 17.2 percent from last year. It still expects double-digit earnings growth in 2009, Brotz said.

The FDA appears to be issuing more complete response letters as a way of buying the heavily burdened agency more time to complete its drug reviews, Nalbone said.