Patients with the heart condition atrial fibrillation have nearly twice the risk of having a stroke in the first month after starting to take the anti-clotting drug warfarin compared to non-users, it has been found.

The research found that the risk was higher in the first week after patients started to take warfarin. In contrast, once the first 30 days had passed, the stroke risk was halved in patients taking warfarin compared to non-users.

However, it is felt that this stroke risk may only affect a small number of patients.

Over 70,000 patients took part in the study carried out by researchers in Canada and the US.

Atrial fibrillation (AF) is where people have an irregular and sometimes abnormally fast heartbeat

Having it increases your risk of suffering a stroke, and warfarin is often prescribed to help reduce this risk, However, the drug does have side effects, which, ironically, include inducing stroke.

Trials of other anti-clotting drugs have suggested that there might be an increased risk of a stroke when patients change from the trial drugs to warfarin.

Also, scientists say warfarin can lead temporarily to a hypercoagulable state - where the blood becomes more sticky and more likely to develop dangerous clots.

Dr Laurent Azoulay, who led the research, 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 future studies should identify the small group of patients who may be at risk.

"However, the results of our study suggest that physicians should be vigilant when initiating warfarin, particularly in the first week of use," he said.

The senior author of the study, Professor Samy Suissa, said while the findings needed to be confirmed in other settings, it should also be investigated whether the newer anticoagulant drugs recently introduced to the market also carry this early stroke risk.

In the meantime, he suggests that a 'bridging strategy' using heparin – an injectable anticoagulant – at the start of warfarin treatment could be considered as a way of reducing the increased risk seen in the first 30 days of warfarin use.

The study, funded by the pharmaceutical firms Bristol Myers Squibb and Pfier, is published in the European Heart Journal.