The number of stroke patients that can be treated with medicine to destroy blood clots is set to double, after a team of German scientists speaking at the 6th World Stroke Congress being held this week in Vienna found the therapy works even four and a half hours after a stroke.

Every year, about 20 million people suffer strokes, which are mostly caused by blood clots.

Werner Hacke, a neurologist at the University of Heidelberg, said Thursday in Vienna that his team had found out that the trombolytic drug Alteplase can be used to break down the clots longer after the stroke than was previously known.

Until now, there was a consensus that patients had to receive the drug within three hours, Hacke said.

According to the scientist, only 25 per cent of patients visit a hospital within three hours of a stroke. Therefore, only four per cent of patients in the United States and Europe are treated with the blood clot buster.

Removing the clots prevents further brain damage by restoring the blood flow.

The new findings, which have been published in the New England Journal of Medicine, could lead to a doubling of the number of stroke victims that can receive this therapy.

However, treatment within 90 minutes was "twice as effective as after three hours," Hacke said.