The National Stroke Network is disappointed some stroke sufferers are being denied the life-changing drug alteplase.

New research in the New Zealand Medical Journal shows four DHBs won't give patients the drug after hours because trained physicians aren't available.

It must be injected within four hours of the first symptoms.

Professor of Clinical Neurology Alan Barber said 30 percent of those treated with alteplase can usually walk out of hospital completely normal afterwards.

He said it could prevent the long-term effects of a stroke.

"A stroke is devastating. Half the people who have one are either dead or dependent on others for basic activities of daily life like going to the toilet, having a shower, feeding yourself, dressing."

Barber said it must be available to patients at all hours.

"If your're asking if this treatment should be available to all New Zealanders 24 hours a day, absolutely. There aren't that many medical treatments that are really this effective, particularly in something like stroke."

The researchers won't name the hospitals involved.

Nine-thousand people suffer a stroke in New Zealand every year.