A popular antibiotic used to treat a variety of infections may cause a potentially fatal heart rhythm problem in some patients.

Azithromycin, also known under its brand name Zithromax, is prescribed for infections of the ears, lungs, sinuses, skin, throat and reproductive organs.

The drug, which is also sold under the brand names Zmax as Z-Pak in the U.S., is a popular choice because patients can take fewer doses over a shorter period than some other antibiotics.

But the U.S. Food and Drug Administration warned yesterday that the antibiotic can interfere with the heart’s electrical activity.

Those most at risk include are elderly people and those with irregular heart rates - arrhythmias - and people with low levels of potassium or magnesium.

The warning comes after a study last year in the New England Journal of Medicine found a small increase in cardiovascular deaths among people treated with Zithromax compared to those given the antibiotics amoxicillin or ciprofloxacin or no treatment at all.

It found that patients who took Zithromax, made by Pfizer, had higher rates of fatal heart rhythms.

At the time, the FDA urged patients taking Zithromax to not stop taking it without seeking medical advice.

In its warning, the FDA said the drug can alter the electrical activity of the heart, which may lead to a potentially fatal heart rhythm known as prolonged QT interval, in which the timing of the heart’s contractions becomes irregular.

It added that doctors should exercise caution when giving the antibiotic to patients known to have this condition or who have certain risk factors.

The drug could also cause problems in people with torsades de pointes - a specific, rare heart rhythm abnormality.

The drug’s labels have now been updated with information about the possible heart rhythm side effects.

In the UK, the patient information leaflet for the drug states that a potential side effect can be heart rhythm problems.

In a statement, Pfizer stressed that most patients will not be affected by this label change, and that other antibiotics in the same class have similar risks.

‘Patients who are currently prescribed Zithromax should talk to their doctors or healthcare providers if they have questions regarding their treatment,’ the company said.

‘Zithromax has had a well-established benefit risk profile for more than 20 years and continues to be an effective treatment option for patients all over the globe suffering from many types of bacterial infections.’

People taking Zithromax should seek immediate medical care in the event of an irregular heartbeat, shortness of breath, dizziness or fainting, the FDA said.

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By Anna Hodgekiss