The weight-loss drug Meridia (sibutramine) may be associated with abnormal heart rhythms in people with a genetic heart condition known as long QT syndrome and should be avoided, according to researchers writing in the British Journal of Clinical Pharmacology.

The researchers reported in the April journal that a 40-year-old Auckland woman collapsed with a heart attack just 25 days after starting to take sibutramine -- marketed in New Zealand as Reductil.

The patient was a trainee nurse whose husband, finding her unconscious and without a pulse, resuscitated her. She was rushed to hospital where it was found that the diet pill -- together with her undiagnosed genetic heart condition -- had proved a near-fatal combination.

Long QT syndrome is a disorder of the heart's electrical system that leaves people vulnerable to fast, chaotic heartbeats that may lead to fainting. However, few know they have it and it can be difficult to detect, even with an electrocardiogram.

The researchers said assessment of further reports from a New Zealand medical database identified five additional patients who experienced heart palpitations while taking sibutramine.

"Further studies are required, but in the meantime we would recommend that sibutramine should be avoided in patients with long QT syndrome and in patients taking other medicines that may prolong the QT interval," the researchers concluded.