Azithromycin is an azalide, a subclass of macrolide antibiotics. Azithromycin (brand names Zithromax® in Italy; Vinzam® / Zitromax® in Spain; Zmax®; Sumamed®; Aztrin®) is one of the world's best-selling antibiotics, and is derived from erythromycin; however, it differs chemically from erythromycin in that a methyl-substituted nitrogen atom is incorporated into the lactone ring, thus making the lactone ring 15-membered. Azithromycin is used to treat certain bacterial infections, most often bacteria causing middle ear infections, tonsillitis, throat infections, laryngitis, bronchitis, pneumonia and sinusitis.