The U.S. Food and Drug Administration has approved use of a medicine proved to slow the degenerating effects of the rare disease ALS.

It’s the first time in more than two decades the agency approved a treatment for the progressive disease amyotrophic laterals sclerosis – also known as Lou Gehrig's disease -- which affects 12,000 to 15,000 people in the United States.

On Friday, the FDA gave its OK to Mitsubishi Tanabe Pharma America for the drug Radicava, or edaravone. The drug is expected to be available by August.

The FDA said the approval came after a Japanese study displayed the drug’s benefits.

The six-month trial tested more than 100 people and found those who used edaravone “declined less” in terms of daily functioning opposed to those who did not use the drug.

Dr. Eric Bastings, the deputy director of the Division of Neurology Products in the FDA's Center for Drug Evaluation and Research, said the agency quickly encouraged the drugmaker to file to make the drug available in the U.S.

“This is the first new treatment approved by the FDA for ALS in many years,” Bastings said, “and we are pleased that people with ALS will now have an additional option.”