The Food and Drug Administration on Tuesday approved the first drug that prolongs the lives of patients with Lou Gehrig’s disease, although the improvement in survival is slight.

The disease, officially known as amyotrophic lateral sclerosis (ALS), affects about 30,000 Americans, attacking nerve cells in the brain and spinal cord and leaving them hard and shriveled. Over the course of the disease, which averages three to five years, patients’ muscles progressively weaken, leading to paralysis, respiratory failure and death. Five thousand new cases are reported each year.

The newly approved drug, riluzole, is not a cure and only extends survival by an average of three months - “a modest effect,” noted Donna E. Shalala, secretary of Health and Human Services, “but before riluzole there was no therapy for ALS at all.”

“This is the first drug we have ever had that seems to make a difference in the course of ALS a dreadful disease,” said FDA Commissioner David A. Kessler in a statement. “We hope this is just a first step.”

The newly approved drug will be marketed by Rhone-Poulenc Rorer Inc., based in Collegeville, Pa., under the name Rilutek. The company said it would be available by prescription in as little as six weeks. The drug’s side effects include weakness, nausea, vomiting and elevated levels of liver enzymes, the FDA said.

Two other companies, Cephalon Inc. of West Chester, Pa., and Tarrytown, N.Y.-based Regeneron Pharmaceuticals, are currently developing drugs to fight ALS. Regeneron has licensed its drug to California-based Amgen Inc.

The disease gained its familiar name when it struck down the New York Yankees baseball legend. It also afflicts physicist Stephen Hawking, who is confined to a wheelchair.