TRENTON, N.J. — U.S. regulators have approved the first new drug in a decade for Parkinson’s disease, a chronic neurological disorder that causes tremors and movement difficulties.

The Food and Drug Administration said that it has approved Xadago for use when a patient’s regular medicines aren’t working well.

The pill was tested in two six-month studies that included about 1,200 patients taking a standard treatment, levodopa.

According to the FDA, adding Xadago to levodopa decreased periods with troubling symptoms such as involuntary muscle movement. Patients who took Xadago also had better control of movement compared to groups that got levodopa and dummy pills.

Roughly 1 million Americans and up to 10 million people worldwide have Parkinson’s. It generally strikes the elderly, affecting 1 percent to 2 percent of those over age 65.

Parkinson’s is a progressive disease with symptoms worsening over time. Levodopa, which most patients take, can become less effective as the disease progresses. So additional medicines are then needed.

Milan, Italy-based Newron Pharmaceuticals, which has a U.S. subsidiary in Morristown, N.J, developed Xadago, known chemically as safinamide. It was approved in the European Union in 2015.

Newron’s partner, US Worldwide Meds of Louisville, Ky., will market Xadago in the U.S. That company said the drug’s list price without insurance will be $670 for a 30-day supply of either the 50- or 100-milligram dose.

Possible side effects can include involuntary movement, nausea and insomnia. The FDA said Xadago should not be taken by people with severe liver problems or those taking opioid painkillers and certain antidepressants.