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.
Warfarin, originally developed as a rat poison, is the most commonly prescribed oral anti-clotting agent in the UK.
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 ischaemic stroke - one caused by the blockage of blood flow to the brain - by 71%.
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 Dr 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 “hypercoagulable” 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 anticoagulants.
Rapid depletion of protein C in particular might lead to a temporary hypercoagulable state, said the scientists.
Co-author Professor Samy Suissa, also from McGill University, said: “While these findings need to be confirmed in other settings, it would be imperative to also investigate whether the newer popular anticoagulants also carry this early risk.”
He suggested offering an injected anti-clotting drug, heparin, to patients starting warfarin treatment to counteract the increased risk of stroke. Heparin is routinely given to hospital patients who have undergone surgery.
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