Doctors are being warned to take greater care in prescribing high-risk blood-thinning medications after a series of compensation claims from patients who suffered strokes and were left with serious disabilities.

The Accident Compensation Corporation has accepted 19 treatment injury claims since the start of last year regarding warfarin, an anti-clotting drug.

Two per cent of warfarin patients suffer major bleeding, which can cause strokes and be fatal.

Doctors sometimes in addition prescribe aspirin - another anti-clotting drug - to reduce the risk of life-threatening clots in patients with mechanical heart valves, acute heart disease, and following the placing of a heart-artery stent.

In that case, the risk of major bleeding is significantly higher than from warfarin alone.

The Health Ministry's Medsafe unit last August reminded doctors that the two-drug combination increased the risk of major bleeding, after noting ACC had received a number of claims from patients on warfarin and aspirin.

Medsafe has this week reiterated its concerns over anti-clotting drugs and the risks, particularly of strokes, in its Prescriber Update publication.

"Despite [last year's advice] ACC continues to receive reports of serious injury in patients receiving warfarin, including cerebrovascular accidents that have resulted in severe disability."

It said patients on warfarin must be regularly monitored by blood tests for clotting levels.

Medsafe's group manager, Dr Stewart Jessamine, said yesterday that when doctors prescribed warfarin, they should check whether the patient was already taking aspirin.

Wellington medical specialist and ACC adviser Dr Phillip Matsis said prescribing a combination of warfarin and aspirin was increasingly common as more patients with atrial fibrillation - a type of abnormal heart rhythm - were going on to develop heart disease.

ACC said adding aspirin to warfarin might benefit some patients, although warfarin alone might be sufficient for patients who were at risk of heart artery disease or who had stable heart artery disease without valves or stents.