From the Editors of The Daily Cat
Are you sure it's your cat's breath? A bad odor that seems to come from the mouth may also be emanating from the cat's ears or other areas around the head. In any case, a visit to the veterinarian is in order.
Bad breath is a sign that there could be an underlying health problem that should be examined by your veterinarian. Usually odors come from bacteria. The source of bacteria could be an accumulation of plaque and tartar on the teeth and would require a professional cleaning by your veterinarian. Cats, just like humans, will collect plaque and tartar on their teeth and require regular dental maintenance, including daily brushing and regular professional cleaning and polishing.
In rare situations, there could be something stuck to your cat's teeth, or to the roof of its mouth, causing a bad odor. If your cat has recently been into any table scraps (a major no-no), such as chicken bones, a bone could be stuck in its mouth.
In any case, you should have your cat examined by your veterinarian to determine the source of mysterious odor.