'Our results suggest that 320 cases a year of sudden cardiac death can be attributed to QTc-prolonging medication in the Netherlands and, by extrapolation, around 9,000 in Europe and 6,000 in the USA.'

A study in the Netherlands found they were associated with a three-fold increased risk of sudden death due to cardiac arrest.

Two of the drugs are the antibiotics erythromycin and clarithromycin.

Others on the risk list are cisapride and domperidone, used to treat gastro-intestinal conditions, and the anti-psychotic medications chlorpromazine, haloperidol and pimozide.

All prolong the heart's QTc interval -- a measurement of the electrical activity linked to the contraction of heart muscle cells. Drugs that increase the QTc interval can cause life-threatening disruptions of heart rhythms.

The findings emerged from a study of 775 cases of sudden heart death.

Researchers found that the seven drugs were probably responsible for 320 of these deaths. This equated to about 15,000 deaths per year across Europe and the United States.

But the study's senior author, Dr. Bruno Stricker, from the Erasmus Medical Centre in Rotterdam, said that although the findings were significant, it was important to keep them in proportion.

It was normal to expect one or two sudden cardiac deaths per thousand of the population each year in Western countries. The risk for people taking the drugs rose to around three per thousand.

"These drugs are vital treatments for serious conditions in many cases, so it is essential that patients should not stop taking them on their own initiative," said Dr. Stricker, who is also a senior medical officer at the Inspectorate for Healthcare in The Hague.

"If they are concerned, they should talk to their doctor."

The drugs have all previously been implicated in abnormal heart rhythms (arrhythmia).

But the new study is thought to be the first to investigate links with sudden death.

The findings appeared today in the European Heart Journal.

Dr. Stricker said, "Past use was not associated with increased risk. Although the antibiotics have been reported to be linked to sudden cardiac death, we found no statistically significant increase in our study, although that may have been due to the limited number of cases."

He said there were potential limitations to the study. Some misclassifications could not be excluded, and a few deaths may have been missed. It was also possible that not all the deaths were heart-related.

But these confounding factors were unlikely to explain the strong association found between the drugs and sudden cardiac death.

"Nevertheless, our results suggest that 320 cases a year of sudden cardiac death can be attributed to QTc-prolonging medication in the Netherlands and, by extrapolation, around 9,000 in Europe and 6,000 in the USA," said Dr. Stricker.