NEW YORK — Researchers using an experimental drug have succeeded for the first time in slowing the progression of a type of multiple sclerosis in which unpredictable attacks leave victims increasingly disabled.

The medication, like another experimental drug discussed on Monday at the annual meeting of the American Neurological Association in San Francisco, also reduces the number of MS attacks.

The drugs are aimed at a relapsing kind of MS that affects 75,000 to 140,000 Americans. In all, 250,000 to 350,000 Americans have MS.

People with relapsing MS suffer bouts of symptoms, including fatigue, impaired vision, loss of balance and coordination, slurred speech, tremors and partial or complete paralysis. Patients recover at least partially during the weeks or months after each episode but become progressively disabled.

Doctors now treat many people with relapsing MS with a drug called interferon beta 1B, or Betaseron. This drug, approved by the Food and Drug Administration just last year, reduces the frequency and severity of attacks.

The new results are welcome because the experimental drugs may provide options, said Stephen Reingold, vice president for research and medical programs at the National Multiple Sclerosis Society.

“From our perspective, this is terrific,” he said, adding that the new results must be analyzed completely, published for scrutiny by experts and submitted to the FDA for evaluation as soon as possible.

One study found that a drug called copolymer I reduced the number of MS attacks. Researchers had 125 patients inject themselves daily with the drug, and 126 inject themselves with a placebo.

Over two years, patients using the drug had a total of 160 MS attacks, compared with 210 in the placebo group.

The other study found that a substance called interferon beta-1a can prevent or delay increases in long-term symptoms, as well as reduce the number of attacks, said principal investigator Dr. Lawrence Jacobs.

No previous drug has shown an ability to delay the progression of symptoms over the long term, Reingold said.