Bad breath is common in cats and is generally termed oral malodor (halitosis). In general, it can be attributed to the protein-rich diet leaving food debris in the mouth, or products of metabolizing protein called volatile sulfur compounds. These volatile sulfur compounds are made up of food debris, saliva, blood, and exfoliated epithelium. However, malodor can also be a sign of a larger problem. Gastrointestinal, lung, and systemic diseases can all result in oral malodor. Also, gingivitis, an inflammation of the gums, and periodontitis, an inflammation and infection of the bones and other tissues that support the teeth, can be detected by oral malodor. These conditions can be harmful to companion animals, causing pain, loss of appetite, and infections that can spread to other body systems. A consultation with your veterinarian would be the best approach to assess the cause of your cat's breath odor. Regular dental care will also help prevent oral malodor. Also, do not overlook the fact that your cat is hissing at you. This behavior may indicate that she is uncomfortable or in pain.