Oxley said the most important thing for people to understand is that large strokes are very treatable. Doctors are often able to reopen blocked blood vessels through techniques such as pulling out clots or inserting stents. But it must be done quickly, ideally within six hours and no longer than 24 hours, Oxley said: “The message we are trying to get out is if you have symptoms of stroke, you need to call the ambulance urgently.”