Recent research has shown that the popular antibiotic known as azithromycin and sold under the brand name Zithromax, Zmax and Z-Pac might cause fatal heart-related side effects. The FDA characterizes the side effect risk as small but notes that Zithromax can cause QT interval prolongation. QT interval prolongation can indicate a type of irregular heartbeat that could be potentially fatal. Symptoms of this irregular heartbeat include palpitations, fainting, cardiac arrest and even sudden death. If you or a loved one was injured after taking Zithromax, consult with an experienced attorney to discuss your legal options.