For cat bad breath. I took my 14 year old cat in to vet - he was doing poorly. Losing weight, poor appetite, really bad breath. Did blood tests - but none of the usual ailments present. The vet told me that with bad teeth - they have bacteria in their mouth that is picked up by the blood and carried around their body.
As nothing conclusive came up in bloodwork, I decided to work on his teeth. I started to brush his teeth daily with a small amount of baking soda. I put a little bit on a child's toothbrush and just brush gently but firmly.
He doesn't particularily like it. He also doesn't rush to get the taste out of his mouth either. I think the time that the baking soda is in his mouth it is killing bacteria.
After doing this for a couple of days, his appetite returned. He seemed to have more energy and his fur seems nicer. His breath isn't terrible.
I should note I might have tried something added to his water, but he will only drink out of the tap.