Ten thousand stroke victims a year are being given the wrong dose of life-saving drugs because doctors are merely guessing their weight, research has found.
A failure to properly weigh patients before they are given clot-busting drugs in NHS hospitals means too many people are receiving the wrong dose - often too little of the drug.
Researchers behind the study said A&E departments should be forced to stock beds capable of weighing patients suffering acute stroke.
The study of more than 200 patients, published in the journal Stroke, was carried out by experts at the Institute of Cardiovascular Research at Royal Holloway, University of London and St Peter's hospital in Surrey.
It found doctors were poor at guessing the weight of patients, particularly the heaviest people (weighing over 80kg, or 12st 8lbs)).
Overall, approximately 20% of patients received an incorrect dose of drugs (about 11% too little and 8% too much).
Around one in five of the heaviest patients received a 10% incorrect (usually too little) dose of medicine.
These patients suffered the worst outcomes, and were more likely to suffer cerebral bleeding and a poor response to the drugs.
Clot-busting medication - known as thrombolysis - works by dissolving blood clots and restoring the flow of blood to the brain.
Lead researcher Professor Pankaj Sharma, from the Institute of Cardiovascular Research at Royal Holloway, said: "Our study has revealed that patients are being given the wrong dose of up to 10% of the amount they should receive which is resulting in poorer outcomes.
"This was the case in one in five of the heaviest patients, which made up a third of the studied population.
"If we were to extrapolate this data to the approximately 150,000 individuals that suffer a stroke annually in the UK then it would mean around 10,000 stroke victims per year are receiving the wrong dose, potentially impacting on their final functional outcome."
Of the need for beds that weigh, Professor Sharma added: "I would struggle to think of an alternative intervention for stroke that would benefit so many patients, in so short a time, for so little money."