Dog breath shouldn’t be smelly breath. If you are starting to smell unpleasant odours coming out of your dog’s mouth, they may have a problem. Bad breath is normally the result of bacteria build-up in your dog’s mouth, lungs or stomach. If you’re only smelling it infrequently, it may be the result of something they’ve eaten. But if their breath always smells bad, it could be time to take them to see the vet.