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, Daily Mail reports.

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).

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  by 71 per cent.

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