People with an irregular and often abnormally fast heartbeat, known as atrial fibrillation, have a 71% greater risk of suffering a stroke in the first 30 days after starting to take the warfarin, an anti-clotting drug, compared to non-users.
Once the first 30 days had elapsed the risk of stroke was halved in patients taking warfarin compared to non-users.
The study analysed data from 70,766 patients aged 18 and over, with 2% experiencing a stroke during that time.
Researchers insist that the study should not deter users, as it is likely to affect only a small number of patients. But doctors should be vigilant when initiating warfarin, they say, particularly during the first week of use.
Read more at McGill University (Montreal, Canada)