Demeclocycline is tetracycline which lacks the methyl substituent at position 7 and in which the hydrogen para- to the phenolic hydroxy group is substituted by chlorine. Like tetracycline, it is an antibiotic, but being excreted more slowly, effective blood levels are maintained for longer. It is used (mainly as the hydrochloride) for the treatment of Lyme disease, acne and bronchitis, as well as for hyponatraemia (low blood sodium concentration) due to the syndrome of inappropriate antidiuretic hormone (SIADH) where fluid restriction alone has been ineffective. It has a role as an antibacterial drug and a geroprotector.