Treatment depends on your vet's diagnosis. If plaque is the culprit, your dog might require a professional cleaning. If it's an issue of diet, you might have to change your dog's regular food. If the cause is gastrointestinal or an abnormality in your dog's liver, kidneys, or lungs, please consult your vet about steps you should take. Many people assume that bad breath in dogs, especially at a certain age, is a "given." But that's not the case. In fact, being proactive about your pup's oral health will not only make your life together more pleasant, it's smart preventive medicine.
I recommend the CET products:
Those chews are good, and so is this spray:
Hope this information helps! Best of luck.