Zithromax is an antibiotic drug. Antibiotics fight infections caused by bacteria. is Zithromax used to treat many different types of bacterial infections, such as bronchitis, pneumonia, tonsillitis, skin infections, ear infections, and sexually transmitted diseases. Zithromax, Generic Zithromax, Azithromycin may also be used for purposes other than those listed in this medication guide. Zithromax may also be used for purposes other than those listed here. Zithromax is used to fight opportunistic infections in people with HIV.