A May 2012 New England Journal of Medicine study that suggested Zithromax could cause heart problem has come under criticism.

The NEJM Zithromax study consisted of an analysis of data from millions of patients. Researchers found that patients taking Zithromax had an increased risk of developing heart problems, including cardiac arrhythmias and sudden heart attacks. This heart risk was higher for people taking Zithromax than for people taking other antibiotics, and even higher in patients with an existing history of heart problems.

Within 24 hours of the study’s publication, the FDA announced it was evaluating data related to Zithromax heart problems.

The proposed mechanism for causing Zithromax heart problems is that Zithromax could cause a type of cardiac arrhythmia or abnormal heart rhythm called a prolongation of the QT interval. This could theoretically cause the heart’s two pumping muscles, the ventricles, to fall out of step with each other, eventually hydraulically locking each other. Doctors call this situation a “widowmaker” heart attack, since it causes the heart to stop almost instantly. Even for a heart attack, this type of misfire has a dismal survival rate.

But the NEJM Zithromax heart study has been criticized in several letters to the editor. In a medical journal like NEJM, a letter to the editor typically come from experts in the field and may even undergo peer-review like a proper article. They are still generally, however, opinions without studies to back them up.

Several physicians have charged that since the study is observational, it is not absolute proof that Zithromax can cause cardiac arrhythmias. This is a downside to observational experiments over clinical studies. However, clinical studies can suffer from problems of their own. This includes smaller sample size, since it is more costly to perform clinical studies.

Additionally, it is easier to get into an ethics issue in clinical studies, since researchers seek to control the variables directly. This means that if researchers suspect a drug is causing a problem, they must halt their study, compromising their data. Since observational studies of this type are done after the fact, there is less of an ethical issue.

Other letters didn’t criticize the type of study, but presented arguments that the risks of Zithromax heart problems are small compared to the benefits of the drug. This included a letter from a Pfizer employee. Pfizer is the manufacturer of Zithromax.

Still another letter suggested that Zithromax, and antibiotics in general, are over-prescribed, and that the heart deaths found in the study could have been avoided by judicious use of antibiotics. Antibiotic overuse is currently a major issue in medicine, as overuse is linked to antibiotic resistance.

Zithromax Lawsuit Plaintiffs Say Heart Problems are Real

While the various letters portray different reactions to the Zithromax heart study, concerns remain that Zithromax could cause heart attacks and other problems.

A number of Zithromax lawsuits have been filed by patients alleging that their heart attack — or their family member’s heart attack — was caused by Zithromax. These Zithromax lawsuit plaintiffs cite the research published in the New England Journal of Medicine to support their claims.

Plaintiffs accuse Pfizer of failing to adequately disclose the heart risks associated with Zithromax use, putting patients’ safety behind their desire for profit.