People should take aspirin "immediately" if they believe they have suffered a minor stroke in order to reduce their risk of having another, more major stroke, experts have said.
Aspirin is already prescribed to people who have had a stroke or transient ischaemic attack (TIA, often called a 'mini-stroke') to prevent further strokes after they have been assessed in hospital and in the longer-term.
This is thought to reduce their subsequent stroke risk by about 15% by reducing the risk of blood clots forming and thinning the blood.
But a new study, led by a team of researchers from the University of Oxford, found that aspirin is most effective when received sooner.
Immediate self-treatment with aspirin after initial "warning events" associated with stroke was found to reduce the early risk of a fatal or disabling stroke by 70-80%.
Lead researcher Professor Peter Rothwell, a stroke expert from the University of Oxford, explained: "The risk of a major stroke is very high immediately after a TIA or a minor stroke (about 1000 times higher than the background rate), but only for a few days.
"We showed previously...that urgent medical treatment with a 'cocktail' of different drugs could reduce the one-week risk of stroke from about 10% to about 2%, but we didn't know which component of the 'cocktail' was most important.
"One of the treatments that we used was aspirin, but we know from other trials that the long-term benefit of aspirin in preventing stroke is relatively modest.
"We suspected that the early benefit might be much greater. If so, taking aspirin as soon as possible after 'warning symptoms' event could be very worthwhile."
The team therefore revisited patient data from previous trials on aspirin as a treatment after stroke.
They found that almost all of the benefit of aspirin in reducing the risk of another stroke was in the first few weeks, and that aspirin also reduced the severity of early strokes.
Rather than the 15% overall reduction in longer-term risk reported previously in trials, aspirin reduced the early risk of a fatal or disabling stroke by about 70-80% over the first few days and weeks.
Professor Rothwell said: "Our findings confirm the effectiveness of urgent treatment after TIA and minor stroke - and show that aspirin is the most important component. Immediate treatment with aspirin can substantially reduce the risk and severity of early recurrent stroke.
"This finding has implications for doctors, who should give aspirin immediately if a TIA or minor stroke is suspected, rather than waiting for specialist assessment and investigations."
The symptoms of a TIA are the same as those of a stroke, but unlike a stroke they only last for a few minutes or hours.
The main symptoms can be remembered with the word FAST: Face-Arms-Speech-Time.
Face – the face may have dropped on one side, the person may not be able to smile, or their mouth or eye may have drooped.
Arms – the person with suspected stroke may not be able to lift both arms and keep them there, because of arm weakness or numbness in one arm.
Speech – their speech may be slurred or garbled, or the person may not be able to talk at all, despite appearing to be awake.
Time – it's time to dial 999 immediately if you notice any of these signs or symptoms.
Professor Rothwell added that the findings also have implications for public education.
"Public information campaigns have worked in getting more people to seek help sooner after a major stroke, but have been less effective in people who have had minor strokes or TIAs.
"Many patients don't seek medical attention at all and many delay for a few days. Half of recurrent strokes in people who have a TIA happen before they seek medical attention for the TIA.
"Encouraging people to take aspirin if they think they may have had a TIA or minor stroke - experiencing sudden-onset unfamiliar neurological symptoms - could help to address this situation, particularly if urgent medical help is unavailable."
Dr Dale Webb, director of research and information at the Stroke Association, added: "A TIA is a medical emergency and urgent neurological assessment must always be sought.
"We welcome this research which shows that taking aspirin after TIA can dramatically reduce the risk and severity of further stroke. The findings suggest that anyone who has stroke symptoms, which are improving while they are awaiting urgent medical attention can, if they are able, take one dose of 300 mg aspirin.
"The research findings are also timely, as the stroke community is currently working to develop a new set of national clinical guidelines on stroke."
The research is published is the Lancet journal.