Demeclocycline is a tetracycline antibiotic which was widely used to treat bacterial infections. It is also used to treat the syndrome of inappropriate antidiuretic hormone (SIADH). SIADH is a specific hormonal imbalance which causes water retention and more concentrated urine. Demeclocycline is administered for SIADH because it antagonizes the antidiuretic hormone and increases the concentration of water in the urine.