Most often, bad breath in dogs indicates dental issues like swollen gums, tartar, and plaque. However, if your dog has bad breath for a long time and even after taking care of its dental hygiene, there is no visible improvement; it may be a serious issue. Chronic bad breath may be an indication of gastrointestinal and respiratory problems. Besides, it can also be an indication of diabetes and liver and kidney problems. These diseases can make your dog's breath smell metallic, and in this case, you should contact your vet immediately.