Epianhydrotetracycline is a secondary degradation product formed by epimerisation of tetracycline and dehydration at the C6 position to aromatise the B ring. Epianhydrotetracycline is an important standard for monitoring tetracycline stability. Although the degradation is associated with a loss of antibiotic activity, epianhydrotetracycline is considered biologically active and is thought responsible for aspects tetracycline toxicity.