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Updated from 12:03 p.m. EDT

Shares of



tumbled 36% after the drug maker said one of its drugs has no effect in treating attention deficit hyperactivity disorder.

Cephalon finished Monday's session down 22 7/16, or 36%, at 40 5/6.

According to Cephalon, the information emerged from a clinical study of Provigil, which treats the sleeping disorder narcolepsy. The 113-patient, double-blind, placebo-controlled trial compared 100 milligram and 400 milligram doses of Provigil to a placebo. Frank Baldino, chairman and chief executive, said that the company is "disappointed" with the study's results.

Even so, he said in a statement that the company, based in West Chester, Penn., still plans to pursue label expansion of Provigil in the areas of excessive sleepiness and fatigue.

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The drug was approved by the

Food and Drug Administration

in 1998 as a once-a-day therapy.

Two weeks ago, Cephalon announced its planned

acquisition of



, a developer of therapeutic applications. At the time, the deal was valued at $444 million. However, Cephalon's Monday dive cuts the value of the deal by 42%, to $256 million.