BOSTON, AUG. 12 -- Researchers reported today that an experimental drug could be the first safe and effective treatment for the most common form of the crippling disease multiple sclerosis, but they cautioned that more research is needed to be sure.

"It's very promising," said Dr. Murray B. Bornstein of the Albert Einstein College of Medicine in New York, who reported the results of a preliminary study in The New England Journal of Medicine. "But we still have a lot of work to do."

The study found that the synthetic drug known as Copolymer 1 or Cop 1 appears to reduce the likelihood that multiple sclerosis patients will suffer attacks and seems to slow the progression of disease in patients with milder cases.

"These data strongly suggest it is effective and it is safe," Bornstein said.

While emphasizing that further study is needed, Dr. Byron Waksman of the National Multiple Sclerosis Society said the results are encouraging.

But in a statement, the society warned that a " 'black market' supply of Copolymer I may be available. If so, it is important to recognize that there can be no assurance of source, quality, therapeutic effectiveness, or safety . . . ."

About 250,000 Americans suffer from multiple sclerosis, which is the foremost crippler of young adults. The disease generally strikes between the ages of 20 and 40. Victims usually experience weakness, numbness, coordination and balance problems, blurred vision and slurred speech.

Some researchers believe multiple sclerosis is caused by a malfunction in which the victim's immune system attacks the coating around nerve fibers.

Cop 1, developed at the Weizmann Institute of Science in Rehovot, Israel, is a synthetic version of a protein in the nerve coat that may trigger the malfunction of the immune system. While researchers are uncertain how Cop 1 may work, it could initiate a harmless immune response that preempts the harmful response.