Stroke risk 'higher at start of warfarin treatment'

"Warfarin doubles risk of stroke in first week for irregular heartbeat sufferers," reports The Daily Telegraph.

Warfarin is a drug known to reduce the risk of stroke in people with atrial fibrillation, but the results of the study this headline is based on suggest that particular care should be taken when treatment is started.

Atrial fibrillation is the most common abnormal heart rhythm. This uncoordinated heart function causes blood to not be fully expelled with each heartbeat. As a result blood clots can form, and if a clot reaches the brain, it can block an artery, causing a potentially fatal ischaemic stroke.

Warfarin reduces the likelihood of blood clots forming. Previous studies have suggested that starting warfarin is associated with an increased risk of ischaemic stroke, and the researchers wanted to see if this was true.

The researchers compared people with atrial fibrillation who had a stroke with people who didn't have a stroke. They found that during the first 30 days of treatment, warfarin was associated with a 71% increased risk of stroke, with a peak in risk during the first week of treatment. However, after 30 days of treatment, warfarin was associated with a reduced risk of stroke.

The researchers suggest that the way that warfarin works causes a short period of excessive blood clotting.

However, in this study, people who took warfarin were compared with people who previously took no antithrombotic therapy. It is likely that the people who took warfarin were at higher risk of having a stroke than people who took no antithrombotic therapy.

Where did the story come from?

The study was carried out by researchers from McGill University and the Jewish General Hospital in Montreal, Canada, and Princeton University in the US.

It was funded by Bristol-Myers Squibb and Pfizer Inc, two pharmaceutical companies that make anticoagulant drugs. Two of the researchers also separately declared work for pharmaceutical companies that manufacture anticoagulant drugs in their conflicts of interest statement.

It was published in the peer-reviewed European Heart Journal.

What kind of research was this?

This was a nested case-control study. A nested case-control study compares cases and controls from a defined cohort (group) of people. In this study, people who had an ischaemic stroke (a stroke caused by something that stops blood flow to the brain) were compared with up to 10 people who didn't have a stroke.

The second group were matched on the basis of age, sex, when atrial fibrillation was diagnosed, and how long people had had atrial fibrillation from a group of people with the condition in the UK.