A view shows the U.S. Food and Drug Administration (FDA) headquarters in Silver Spring, Maryland August 14, 2012. REUTERS/Jason Reed/File Photo

(Reuters) - The U.S. Food and Drug Administration said it had approved the first generic version of Xyrem, Jazz Pharmaceuticals Plc’s drug to treat patients with narcolepsy, a kind of sleeping disorder.

The approved generic is developed by Roxane Laboratories Inc, the FDA told Reuters.

Jazz Pharma’s shares, which lost about 22 percent of their value in 2016, were down 6.6 percent at $108.00 in after-hours trading on Tuesday.

Xyrem is the only approved treatment for both excessive daytime sleepiness and cataplexy in patients with narcolepsy, a chronic neurological disorder.

Cataplexy, one of the primary and the most debilitating symptoms of narcolepsy, causes a sudden loss of muscle control, leading to physical collapse. (bit.ly/2jHzIS9)

About 70 percent people with narcolepsy suffer from cataplexy, the FDA said.

Xyrem generated sales of $816.4 million in the nine months ended Sept. 30, representing about 75 percent of Jazz Pharma’s net product sales.