If the bad breath is caused by halitosis, then niacin can be of great help. Niacin aids in the production of stomach acid. Niacin itself helps in digesting protein, carbs and fat. It must be the kind that causes flushing to work. Take it after a full meal and you will not flush. No-flush niacin won't work. Niacinamide won't work and inositol hexanicotinate won't work.
Must be straight Niacin. You will notice a nice anti-anxiety effect too. Usually when people with constipation take niacin they notice complete relief from irregularity, bad breath and bloating.
Try 100mg after meals. If that doesn't help, increase the next day to 200mg after meals. They come in 100 and 500mg tablets. Swiss makes it. Natural Factors too. Don't take 500mg. Work up to that only if you need it. If you take more than your body is asking for, you will get dizzy. I got dizzy at 700mg, so I just take 500mg after large meals and half a tablet after a small meal. No problems with that.