Azithromycin is an azalide, a subclass of macrolide antibiotics. Azithromycin (brand names Zithromax in Italy and the United States; Vinzam / Zitromax in Spain; Zmax; Sumamed; Aztrin, Zitrocin, Azibiot) 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. It is also effective against certain sexually transmitted infectious diseases, such as non-gonococcal urethritis and cervicitis. Recent studies have also indicated it to be effective against late-onset asthma, but these findings are controversial and not widely accepted as of yet.