SN instructed patient and caregiver that Zithromax (Azithromycin) is an antibiotic useful for the treatment of bacterial infections. Most common side effects are diarrhea , nausea, abdominal pain and vomiting. Nervousness, dermatologic reactions, and anaphylaxis have been reported. As with all antimicrobial agents, pseudomembranous colitis can occur during and up to several weeks after azithromycin therapy.