GlaxoSmithKline‘s (NYSE: GSK) restless leg syndrome drug Horizant has received U.S. approval for the new indication of treating pain following shingles.

The condition, called postherpetic neuralgia (PHN), is a nerve pain syndrome that follows the healing of an outbreak of herpes zoster — more commonly known as shingles. GSK says that of the 1 million people in the United States who develop shingles each year, an estimated 10 percent develop PHN. This post-shingles pain can last for months, even years.

GSK filed its supplemental new drug application for Horizant last August.

The first commercial sales of Horizant for PHN will trigger a $10 million milestone payment to XenoPort (Nasdaq: XNPT), the Santa Clara, California company that discovered Horizant and co-developed the drug with GSK. The companies entered their partnership on Horizant in 2007, a deal that paid XenoPort $75 million up front with up to $312 million in additional payments for the compound’s progress in RLS and other neuropathic pain indications.

The U.S. Food and Drug Administration in April 2011 approved Horizant to treat moderate-to-severe restless leg syndrome. But the relationship between XenoPort and GSK has not been without problems, and the partners are now clashing in courts over the deal.

In January, XenoPort sought to terminate the partnership claiming GSK had breached the deal because it was not doing enough to maximize Horizant sales and reach sales milestones. Horizant became commercially available last summer. According to XenoPort’s financial reports, GSK generated just $1.3 million in first-quarter net sales for the drug.

XenoPort filed a complaint in California courts in February that seeks to terminate the partnership with GSK. The British pharmaceutical giant, which has its U.S. headquarters in Research Triangle Park, has filed a complaint of its own that seeks a judgement that the company has not breached the deal. The cases remain in litigation.