Most often, bad breath is caused by dental or periodontal disease. Some dogs do not adequately chew their food; thus they don't clean their teeth naturally by the chewing process. Also, some dogs and cats are just prone to dental plaque, tartar and disease. These pets may need more frequent dental cleanings and treatments.
Internal diseases such as kidney disease can also cause bad breath. A physical exam is recommended in order to pinpoint the cause of your dog's bad breath and to ensure he is in good health.