Demeclocycline hydrochloride is an antibiotic isolated from a mutant strain of Streptomyces aureofaciens. It inhibits protein synthesis by binding with the 30S and possibly the 50S ribosomal subunit(s) of susceptible bacteria. Demeclocycline has antimicrobial activity against a wide range of gram-negative and gram-positive organisms. Demeclocycline is indicated in the treatment of infections caused by susceptible strains of the designated microorganisms. Demeclocycline is currently used to treat hyponatremia in patients with the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Demeclocycline mainly attenuates hyponatremia in SIADH by reducing adenylate cyclase 5/6 expression and, consequently, cAMP generation, AQP2 gene transcription, and AQP2 abundance in the renal inner medulla, coinciding with a reduced vasopressin escape response in other collecting duct segments.