Do you brush their teeth? I don't either because mine would never sit still for it but a lot of people do. Really though, if the bad breath is caused by diseased teeth or gums or tartar buildup, nothing you can do at home is going to make any difference - they need to go to the vet. Besides being extremely painful, dental infections can spread all over their body, just like with people. Dental exams and cleanings are not cheap but they really need them when they get to a certain age. My Ragan had really bad breath even though, at age 8, her teeth still looked white and clean with no visible tartar buildup. And the lymph glands in her neck were swollen too. They had to do pre-op bloodwork because of her age and they have to be sedated, which can be risky with Bostons. And even though her teeth looked good, they found a raging infection between her gums and the root of a back molar. They cleaned out the infection, gave her teeth a general cleaning, and pulled one crooked tooth that was rubbing on another one, then gave her big doses of antibiotics. Besides being very painful for the dog, untreated dental problems and infections can greatly shorten their life.