Clot-busting drugs may pose a serious hazard to heart- attack patients by increasing their bleeding risk, research has shown.

A study of more than 40,000 heart-attack patients found some drug combinations tripled or quadrupled the chances of bleeding. Hospital admissions for bleed episodes were associated with a three-fold increased risk of death or recurrent heart attacks.

The researchers warned that great care should be taken when offering heart-attack patients certain anti-clotting treatments. Blood-thinning drugs are routinely used to save the lives of people who have suffered a heart attack. But a balance has to be struck between the medicines' ability to prevent clots blocking arteries and the chances of them causing bleeding from internal ulcers and cuts.

The 16-month study compared combinations of aspirin, clopidogrel, and vitamin K antagonists such as warfarin. Researchers found that, in comparison with aspirin alone, clopidogrel increased the risk of bleeding 1.3 times and vitamin K antagonists 1.2 times. Combinations of aspirin plus clopidogrel raised the risk 1.5-fold, and when aspirin and vitamin K antagonists were combined the risk almost doubled. Pairing clopidogrel and vitamin K antagonists led to a 3.5-fold increase in risk, and triple therapy involving all three drugs quadrupled the risk.

The findings are published today in The Lancet medical journal. The researchers led by Dr Rikke Sorensen, from Copenhagen University Hospital, concluded: "Non-fatal bleeding is... associated with increased risk of recurrent heart attack or death."