A widely-used anti-clotting pill can double the risk of stroke when patients with a faulty heart rhythm first start taking it, a study has found.

Researchers believe warfarin may deactivate two naturally-occurring anti-clotting proteins before its blood-thinning effects are felt. After 30 days, the drug halves the risk of stroke.

Although only a small number of patients are believed to be at risk, the study authors urged doctors to be vigilant.

Scientists carrying out the new research analysed data on 70,766 adult patients diagnosed with atrial fibrillation (AF), which occurs when the heart pumps haphazardly instead of with a steady beat.

AF leads to blood pooling, thereby increasing the risk of clotting and strokes.

Over a 10-year period, 5,519 of the patients suffered a stroke. The study found during the first 30 days of treatment, warfarin increased the risk of ischaemic stroke – one caused by the blockage of blood flow to the brain – by 71 per cent.

The risk peaked on the third day after starting warfarin, when patients on the drug were more than twice as likely to have a stroke than those not treated with it.

Patients with a previous history of ischaemic stroke were 2.5 times more at risk of another stroke during the first month taking warfarin.

Lead researcher Laurent Azoulay, from McGill University in Canada, said: “There is no question that warfarin is highly effective in preventing strokes in patients with atrial fibrillation.

“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.

“Future studies should confirm our results, and identify the small subset 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.”

The findings, published in the European Heart Journal, suggested that patients with “hypercoagul-able” states whose blood clotted easily were most at risk.

Warfarin blocks the action of four clotting factors in the blood. But it also deactivates two other proteins, C and S, which are natural anti-coagulants.

Rapid depletion of protein C in particular might lead to a temporary hypercoagulable state, said the scientists.