Stroke survivors who stop taking their daily prescription of aspirin triple the risk of another stroke within a month, research suggests.
Aspirin is a standard treatment for many stroke sufferers
Aspirin has been shown to cut the risk of recurrent stroke by about 25%.
But Swiss research suggests the protective effect is rapidly lost when aspirin is no longer taken.
If confirmed, the findings may prompt a rethink of the current advice that patients stop taking aspirin in the days before undergoing minor surgery.
Each year in England and Wales, more than 130,000 people have a stroke and, of these, more than 53,000 are recurrent strokes.
The researchers say their work underlines the importance of complying with therapy.
The Swiss researchers focused on 309 patients who had a stroke or a mini-stroke - known as a transient ischemic attack (TIA) - and later went on to suffer a further episode. All of this group had at least inititally been put on long-term aspirin therapy.
The researchers said they found that in 13 cases the patients had stopped taking their aspirin in the four weeks leading up to their latest stroke.
To find out how significant this was, the researchers found another group of 309 stroke patients, who had also been prescribed aspirin but this time had not suffered a recurrent stroke.
In this group, just four patients admitted they had stopped taking their pills during the four weeks surveyed.
From this the researchers, who presented their findings at a conference of the American Stroke Association, calculated that stopping aspirin therapy increased the short term risk of a recurrent stroke by more than three times.
However, they admit more work is needed to firm up their conclusions.
Co-author of the study and director of the acute stroke unit at Lausanne University Dr Patrik Michel told the BBC News website that aspirin reduced the risk of heart problems or stroke by inhibiting enzymes that make tiny particles in the blood called platelets sticky.
But he said the study suggested that stopping aspirin therapy increased the risk of a new stroke above what could be expected for a patient who had never taken aspirin at all - at least for a week or two after having stopped it.
"The findings strongly suggest there is a rebound effect - that blood gets stickier when you stop taking aspirin than it would have been if you had never taken it in the first place," he said.
Dr Michel said the findings raised doubts about the current practice of advising patients to stop taking aspirin days before minor surgery, such as tooth extraction or cataract removal.
This is done because, as aspirin thins the blood, it may increase the risk of bleeding during surgery.
But Dr Michel said the risk from not taking aspirin may be greater.
"Each case must be considered on its merits, but research has already shown that most minor surgical procedures can be done safely even when the patient is still taking aspirin," he said.
Cathy Ross, a cardiac nurse at the British Heart Foundation, agreed that the research raised doubts about the wisdom of ending aspirin therapy before minor surgery.
She said: "It would be a positive thing to get clinicians to be more proactive about looking at the relative risks associated with stroke and heart disease."
A spokesperson for the Stroke Association said: "If aspirin is prescribed to a stroke patient they will most probably need to take it for life.
"As with most medications, people should not stop taking their medication without first discussing it with their doctor or GP."