Medscape - Infection-specific dosing for Zithromax (azithromycin), frequency-based adverse effects, comprehensive interactions, contraindications, pregnancy & lactation schedules, and cost. Zithromax is an antibiotic that fights bacteria. Zithromax is used to treat many different types of infections caused by bacteria, such as respiratory infections, skin infections, ear infections, and sexually transmitted diseases. No fluff, no fear-mongering — just clear, calm guidance on taking Zithromax. When it works fast, when to watch for side effects, why you shouldn’t skip the last pill, and what no one mentions about your gut afterward. It works by killing bacteria or preventing their growth. However, this medicine will not work for colds, flu, or other virus infections. This medicine is available only with your doctor's prescription. This product is available in the following dosage forms: Azithromycin is a semi-synthetic antibiotic of the azalide subclass, derived from the classical macrolide erythromycin by the insertion of a methylated nitrogen atom into the 14-membered macrolactone ring, expanding it to a 15-membered ring. What it is: Azithromycin (Zithromax, Z-Pak)—macrolide antibiotic for community-acquired pneumonia, certain STIs, sinusitis, and some skin infections. Typical dose: Z-Pak: 500 mg on day 1, then 250 mg days 2–5. Overview: Azithromycin is an antibiotic that is used to treat certain bacterial infections. It works by stopping certain types of bacteria from making proteins they need to grow and multiply..