Taking one pill the night before surgery to unclog neck arteries could help save lives, researchers say.
Patients were given the pill the night before their operation
Patients are given the operation to reduce their risk of having a stroke.
But the operation itself carries a small risk of causing sometimes fatal strokes, according to University of Leicester researchers.
Writing in Circulation, they say giving the anti-clotting drug clopidogrel to patients already on aspirin prevented strokes occurring after the operation.
Patients are given carotid endarterectomy surgery to remove the plaque that narrows neck arteries and increases the risk of stroke.
Aspirin is already given to patients during surgery to prevent excess clotting, but some patients still develop clots.
Strokes can happen soon after the procedure because tiny clots called microemboli break off the surface of the cleaned artery.
Around 3% of patients who have the operation either die or have a stroke in the hours following the operation because clots have formed blocking the carotid artery, which supplies blood to the brain.
The Leicester researchers looked at why this might be.
They found that in these high-risk patients, blood platelets, which initiate clotting, were highly sensitive to a chemical called adenosine diphosphate (ADP).
Aspirin appeared to block one clot-activating path in the platelet, but did not affect the activity of ADP.
The researchers looked to see if clopidogrel would have that effect.
A hundred patients, who were already taking aspirin to reduce the risk of blood clots, were studied by researchers.
All were having the operation because they were at high risk of having a stroke.
Around half were given 75mg of clopidogrel, and the rest a dummy drug the night before their operation.
They were monitored using ultrasound scans of the brain and by checking the inside of the artery with a tiny camera at the end of the operation.
In the patients given the drug, researchers saw a 10-fold reduction in the number of patients having more than 20 microemboli within three hours of surgery.
They also found that the clopidogrel did not slow clotting so much that patients experienced excessive bleeding, which can also be dangerous in these patients.
Professor Ross Naylor, who led the research, said: "This is the first study that has ever shown that we might be able to prevent strokes caused by post-operative thrombosis by administering a tablet preoperatively.
"It is now possible to prevent a complication that was previously considered unpredictable and unpreventable."
Dr Anthony Rudd, a stroke specialist at St Mary's Hospital, London, told BBC News Online: "The findings of this study seem to be very encouraging indeed,."
He said larger studies were needed, but if they came to the same conclusion the treatment was likely to be adopted by specialists.
"The only concern is that the treatment could increase the risk of bleeding at the time if the operation, although it doesn't look like that's happened in this study."
The research was funded by The Stroke Association and the University Hospital Leicester NHS Trust.
A Stroke Association spokeswoman said: "Carotid endarterectomy is potentially life-saving surgery for many stroke patients, and we welcome any development towards reducing the risk of post-operative strokes after this procedure.
"However we strongly recommend that patients only take aspirin if prescribed by their doctor."