Drug-maker Abbott Laboratories said on Monday that federal regulators have withheld approval of its experimental long-acting form of the painkiller Vicodin, and there are no indications about when the drug would be approved.

In a statement, the company said it will discuss the matter with the FDA, since it had predicted the approval of Vicodin C-R this quarter. In fact, Abbott stuck to its earlier company earnings forecasts for 2008 and 2009 in spite of the uncertain fate of the pill, which it had expected would generate eventual peak annual sales of $500 million.

Melissa Brotz, an Abbott spokeswoman declined to disclose the contents of the FDA ‘complete response letter’, a document issued when the agency withholds approval until it receives information of some specific type. Brotz also declined to say whether the agency was seeking more clinical trial data or new clinical trials, or when the drug might be approved.

The powerful tranquilizer, ‘Vicodin C-R’ - Controlled Release Vicodin – though not yet hitting the shelves, would allow patients to take fewer pills each day to relieve their pain. Like Vicodin, it contains the narcotic hydrocodone in combination with acetaminophen. Its effect would last for 12 hours, as compared to Vicodin which is taken every 4 to 6 hours.

Phillip Nalbone, an analyst with RBC Capital Markets, said the medicine demonstrated good safety and efficacious results in late-stage clinical trials, and has potential “in every conceivable case where the body is wrecked with chronic pain - fractures, back pain, underlying chronic medical conditions.”

Nalbone also said studies suggest Vicodin C-R would offer significant medical and lifestyle advantages. He added that there would be fewer episodes of ‘breakthrough pain’, and the drug would bring “better night’s sleep.”