(Reuters) - The U.S. Food and Drug Administration (FDA) said it had approved Adamas Pharmaceuticals Inc’s treatment for a side effect caused by a commonly prescribed Parkinson’s drug, sending the shares of the drugmaker soaring in after-market trading.

The company’s shares were up 30.9 percent at $18.84 after the bell on Thursday.

A majority of patients diagnosed with Parkinson’s are treated with levodopa, whose use often leads to dyskinesia - involuntary movements that are non-rhythmic, purposeless and unpredictable.

Parkinson’s disease is a debilitating disorder in which brain cells progressively die causing patients to experience tremors, rigidity, extreme slowness of movement, impaired balance, and difficulties in swallowing and speaking.

Adamas’ Gocovri, previously ADS-5102, is the first drug cleared by the FDA to control levodopa-induced dyskinesia (LID). The long-acting therapy is taken once-daily at bedtime.

An estimated 90 percent of levodopa-treated patients, about 200,000 people in the United States, suffer from LID, the company said.

Fluctuating levels of levodopa result in erratic periods of muscular control and involuntary movements throughout the day, disrupting activity at least half a dozen times a day.

As Parkinson’s progresses, patients are dyskinetic just after taking levodopa but increasingly exhibit “off time”, or worsening symptoms, as it wears off.

These patients have little recourse, other than opting for deep brain stimulation, a surgical procedure that involves blocking electrical signals from targeted areas in the brain.

With Gocovri, which targets both dyskinesia and off time, patients will be able to reclaim about 3.6 hours of their day, CEO Gregory Went said in an interview ahead of the decision.

About 50,000 people are diagnosed with Parkinson’s in the United States each year, according to the National Institutes of Health.

The main ingredient of Gocovri, amantadine, has been available in the market as an antiviral drug for several decades.

The company said the drug is expected to be available in the fourth quarter, and formally launched in January 2018.

Adamas is also testing the drug to treat walking impairment in patients with multiple sclerosis.