Los Angeles ? Stroke sufferers can still benefit from clot-busting medicine even if they receive it an hour or so beyond the current three-hour window after symptoms start, a new study suggests.

The finding could potentially extend treatment to thousands more people each year and prevent many from being left disabled. However, it does not change long-standing advice that stroke victims seek immediate help if they feel sudden numbness or weakness in the face, arm or leg.

“Don’t wait,” said Dr. Larry Goldstein, director of Duke University’s stroke center and a spokesman for the American Stroke Association. “If you think you are having symptoms, call 911.”

The study by European doctors found that the clot dissolver could safely be given up to 4 1/2 hours after the start of symptoms. Results were published in today’s New England Journal of Medicine. Stroke is the nation’s No. 3 killer and the leading cause of disability such as paralysis or speech loss.