Zithromax is a brand of azithromycin, a kind of macrolide antibiotic medication chemically related to erythromycin and clarithromycin. Azithromycin is commonly prescribed to treat certain bacterial infections such as bronchitis, pneumonia, sinusitis, pharyngitis, tonsillitis, ear infections, skin infections, and some sexually transmitted diseases, and works by stopping the bacteria from growing and harming the body any further. It is normally prescribed for a period of 1-5 days and is to be taken at the same time everyday to ensure efficacy. It should be used as directed. Even if you already feel better, the prescription should be finished. If the azithromycin is stopped too soon, the bacteria might become resistant to the antibiotics.