As a result, “it represents a new treatment option that has the potential to be used earlier,” said Dr. Peter Chin, the group medical director of neuroscience at Genentech, who was closely involved in developing the drug.

An estimated 400,000 people have multiple sclerosis in the United States, and about 15 percent have the primary progressive form of the disease.

In the trials that studied the relapsing form of the disease, which involved 1,656 patients, those taking Ocrevus saw a 47 percent reduction in their rate of relapses compared with patients who were taking an existing treatment, Rebif. In the clinical trial for people with primary progressive multiple sclerosis, which involved 732 patients, those on the drug had 24 percent less risk of their disability progressing compared with patients who were taking a placebo.

Ocrevus works by depleting a specific type of a patient’s B cells, which circulate in the blood and are part of the immune system. While they normally help the body fight off infections, they are believed to malfunction and contribute to central nervous system damage in people with multiple sclerosis.

“I think if the safety holds up, it will become the leading M.S. therapy,” said Dr. Steven L. Galetta, the chairman of the department of neurology at NYU Langone Medical Center, who is an expert in multiple sclerosis and who was not involved in the clinical trials. But, Dr. Galetta said, the medical community will be watching to see how the drug performs once it is widely available. The clinical trial showed a slightly heightened rate of tumors in patients with primary progressive multiple sclerosis, which he said needed to be monitored closely. “There can be side effects, but you just didn’t have enough patients initially to confirm that signal,” he said.

Jerrie Gullick, one of the patients who received Ocrevus in the clinical trial, said the drug had significantly slowed the progression of her primary progressive multiple sclerosis since she began taking it about three and a half years ago.

Ms. Gullick, who is 51, had been declining steadily since she learned she had the disease in 2010. At the time she learned she had multiple sclerosis, Ms. Gullick was an active 45-year-old who walked six miles a day between her home in Park Slope and her office in Downtown Brooklyn, where she worked as the chief financial officer of a technology start-up.