Zithromax, a powerful antibiotic manufactured by pharmaceutical giant Pfizer, was the subject of a U.S. Food and Drug Administration (FDA) warning that Zithromax can cause irregular heart activity and result in a fatal heart rhythm.

The active ingredient in Zithromax is azithromycin. First approved by the FDA in 1992, Zithromax is used to treat bacterial infections such as bronchitis, pneumonia, and infections of the ears, lungs and other organs. Its use has since expanded to include middle-ear infections in children and sexually transmitted diseases. As an antibiotic, it is not effective in treating viral infections such as a cold or the flu.

In May 2012, a study in the New England Journal of Medicine reported an increase in cardiovascular death in patients treated with azithromycin compared with patients treated with amoxicillin, ciprofloxacin or no drug.

People with diabetes, a high risk of heart failure, or a previous heart attack are at the most risk of abnormal heart rhythms while taking Zithromax. The authors of the study said doctors should consider prescribing a different drug, such as amoxicillin, for such high-risk patients.

Prompted by the Zithromax study featured in the New England Journal of Medicine, the FDA issued a warning for Zithromax. The study revealed that Zithromax users faced an increased risk of mortality within five days of beginning treatment.

Vanderbilt University researchers discovered that cardiac arrhythmia was 2.5 times more likely in Zithromax users as compared to amoxicillin users.

The FDA warning alerted consumers that Zithromax patients who are at risk for heart problems have an increased risk of arrhythmia, or irregular heart rhythm. At-risk patients include those who have low potassium or magnesium. Patients who have abnormally slow rates or take arrhythmia drugs are also at-risk for Zithromax heart problems.

In March 2013, based on new studies funded by Pfizer in response to the research in the New England Journal of Medicine, the FDA issued a stronger public warning that azithromycin “can cause abnormal changes in the electrical activity of the heart that may lead to a potentially fatal irregular heart rhythm.”

Zithromax Lawsuits

In March 2013, Pfizer noted that in collaboration with the FDA, it had “updated the Zithromax (azithromycin) label with regards to a specific, rare heart rhythm abnormality in patients with risk factors” for heart disease and that “this potential risk is well established in macrolide antibiotics.” This was the first public information supplied by Pfizer about cardiovascular Zithromax complications in the two decades since its introduction.

Because Pfizer did not issue any warnings to the public about the danger of cardiovascular Zithromax side effects until nearly 22 years after it started marketing the drug, families of those who have died while taking Zithromax may be able to file Zithromax lawsuits against the drug maker.

Drug manufacturers are expected to thoroughly test their products and ensure their safety before releasing them to the market. Lawsuits filed against Pfizer over Zithromax will likely allege that the drug maker either knew, or should have known, about the increased risk of cardiovascular death, and failed in its duty to warn the public.