Anhydrotetracycline hydrochloride is a salt of a degradation product of tetracycline, formed by dehydration at the C6 position under acidic conditions to aromatise the B ring. Anhydrotetracycline is an important standard for monitoring tetracycline stability. Although the degradation is associated with a loss of antibiotic activity, anhydrotetracycline is considered biologically active and is thought responsible for aspects of tetracycline toxicity.