A study of over 70,000 UK patients found stroke risk spiked in the first week after the beginning of warfarin treatment compared with patients not taking the drug.
But after 30 days the risk of stroke fell to half that of non-users.
Researchers insisted the findings should not deter use of the drug to reduce the risk of stroke in AF, but said clinicians must remain vigilant in the first weeks after initiating treatment.
A team from McGill University in Montreal, Canada, used 16 years of data on 70,766 adults with AF from UK general practice records.
From this group of patients, they randomly matched 5,519 cases of stroke with 55,022 controls.
Patients who started on warfarin had a 71% higher risk of stroke during the first 30 days of treatment compared with patients taking no anticoagulation drugs. The risk peaked at 2.3-fold higher on the third day after initiation of warfarin.
However, these patients’ risk of stroke halved after 30 days on the drug, compared with non-users.
Authors said the ‘paradoxical’ effect of warfarin in the first days of treatment may be caused by the drug blocking not just clotting factors in the blood, but also deactivating some natural anticoagulants. This may lead to a ‘transient hypercoagulable state’, although further studies are needed to confirm the findings, they said.
Study author Dr Laurent Azoulay, assistant professor at McGill University, said: ‘There is no question that warfarin is highly effective in preventing strokes in patients with AF. Thus, our finding that the initiation of warfarin may be associated with an increased risk of stroke should not deter physicians and patients from using this drug, since this likely affects a small number of patients.’
He said the results of the study ‘suggest that physicians should be vigilant when initiating warfarin, particularly in the first week of use’.
Co-author Professor Samy Suissa suggested that ‘a bridging strategy using heparin – an injectable anticoagulant – at the initiation of warfarin treatment could be considered as a way to reduce the increased risk observed in the first 30 days of use’.