A common heart drug taken by thousands is riskier than previously thought, doctors have warned, after finding more people suffer serious side effects than was apparent in clinical trials.
Warfarin, a drug derived from rat poison, is used to thin the blood in people with heart rhythm problems to prevent blood clots which can cause heart attacks and strokes.
However it is extremely difficult to take because it has a narrow window where it thins the blood enough without being so thin that it can trigger serious bleeds.
It also reacts with lots of other medicines, some foods such as broccoli and spinach and cranberry juice and alcohol so patients have to carefully monitor their levels.
There are around one million people with atrial fibrillation in Britain.
Researchers in Canada have found that when used in real life outside the highly controlled circumstances of a clinical trial, more patients suffer serious bleeds.
In randomised clinical trials between one and three per cent of people on warfarin suffer a bleed per year.
An observational trial of 125,000 people, with an average age of 66, taking warfarin in real life for atrial fibrillation, showed that almost four per cent suffered a bleed per year. The complication rate was even higher during the first 30 days of treatment when almost 12 per cent suffered a bleed.
Over five years, almost 11,000 of the group, around nine per cent, went to hospital with a bleed and 1,963, or almost a fifth of them died.
The findings were published in the Canadian Medical Association Journal.
Lead author Tara Gomes, Institute for Clinical Evaluative Sciences (ICES), Toronto, said rate of major bleeds found in this study was lower than in other observational studies that had not been continued for as long as this one, probably because the complication rate is highest early on after starting warfarin.
However, she added: "The rate of haemorrhage in our study is considerably higher than those reported in randomised controlled trials of warfarin therapy, which have ranged between one per cent and three per cent.
"This difference is likely due to the strict inclusion criteria and close monitoring of patients in clinical trials and the average age of our participants being older than that of patients included in trials.
"The clinical importance of this finding is underscored by the number of patients admitted to hospital for haemorrhage who died either in hospital or shortly after discharge."
She said doctors and patients should be aware of the findings, especially as newer drugs are now becoming available.
Dr Peter Coleman, Deputy Director of Research from the Stroke Association said: “Warfarin is commonly used to treat patients with atrial fibrillation in order to reduce their risk of stroke. However, it is not suitable for everyone and as with many medications, it does carry side effects.
“This research suggests that warfarin could lead to bleeding in the brain in more patients than initially thought. However, more research is needed in this area and warfarin should not be discounted as an effective treatment for atrial fibrillation.
"It’s important that patients already taking warfarin do not stop taking it and anyone concerned should speak to their GP or other health professional for further guidance.”
By Rebecca Smith, Medical Editor, The Daily Telegraph, 26 Nov 2012