Dr. Neil A. Shneider, director of the Eleanor and Lou Gehrig ALS Center at Columbia University Medical Center, said, “The effect is modest but significant.”

He added, “I’m very happy, frankly, that there is a second drug approved for A.L.S.”

The disease kills nerve cells that control voluntary muscles, so patients gradually weaken and become paralyzed. Most die within three to five years, usually from respiratory failure. About 12,000 to 15,000 people in the United States have A.L.S., according to the Centers for Disease Control and Prevention.

Dr. Shneider predicted that patients would be eager to try the new drug. He said several of his patients were already receiving it because they had obtained it themselves from Japan. If more want it, he will prescribe it, he said.

“It’s very safe,” he said.

But he was uncertain about whether he would actually recommend it, because the method of administration is difficult. Patients have to have an intravenous line inserted and left in place indefinitely, which poses an infection risk. The first round of treatment requires a one-hour infusion every day for 14 days, followed by 14 days off. After that, the infusions are given daily for 10 out of 14 days, with 14 days off.

“If it were a pill and easily administered, I’d be less reluctant to use it,” Dr. Shneider said.

He added that the drug will not make patients feel better or stronger. It does not improve symptoms. It can slow the decline, but that is not something a patient will notice.