This morning, Cephalon (NASDAQ:CEPH) reported positive clinical data with its drug modafinil (PROVIGIL) in treating attention deficit hyperactivity disorder (ADHD). PROVIGIL is approved for treating excessive sleepiness associated with narcolepsy, obstructive sleep apnea, and shift work sleep disorder. Based on the clinical data in children with ADHD, Cephalon is filing with the Food and Drug Administration for approval in this market in the fourth quarter.

PROVIGIL is a very important drug in Cephalon's portfolio, accounting for more than 40% of the company's revenue. A major concern is that Cephalon is having to fight in court to protect its PROVIGIL patents against generic versions marketed by Barr Laboratories (NYSE:BRL), Mylan Laboratories (NYSE:MYL), and Teva Pharmaceuticals (NASDAQ:TEVA). Approval of PROVIGIL in the ADHD market would help protect the drug's value.

If PROVIGIL is approved for treating children with ADHD, it would compete with drugs such as Adderall XR from Shire Pharmaceuticals (NASDAQ:SHPGY) and Strattera from Eli Lilly (NYSE:LLY). This represents a large market opportunity, as about 2 million children have ADHD in the U.S. alone.

How much of this market could PROVIGIL capture? In part, it's going to depend on the strength of the clinical data. The data from the phase 3 trials will be released at medical meetings over the next year, and it will be interesting to see how the drug compares with the historical data from the leading competitors.

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Fool contributor Charly Travers doesn't own shares of any company mentioned in this article.