Azithromycin is the first macrolide antibiotic belonging to the azalide group. Azithromycin is derived from erythromycin by adding a nitrogen atom into the lactone ring of erythromycin A, thus making lactone ring 15-membered. Azithromycin is sold under the brand name of Zithromax and Sumamed, and is one of the world's best selling antibiotics. Azithromycin is used for the treatment of respiratory tract, soft-tissue and genitourinary infections.