Updated from 12:03 p.m. EDT

Shares of

Cephalon

(CEPH)

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.

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

Anesta

(NSTA)

, 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.