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Hello. Thanks for the question.
When a cat has an upset stomach, I tend to feed normally, but I'll add a GI protectant. I typically use Pepcid (Famotidine). You can pick this up over the counter and give 5 mg every 12-24 hours.
Trench mouth is a term I would use for bad dental disease. I assume that is what you mean. In that case, the only effective treatment is a dental procedure with x-rays. Any diseased teeth are removed. Sometimes this means removal of all of the teeth. That may be the only curative option in severely affected cats. They are so much more comfortable once the diseased teeth are removed. This may cure the stomach upset that you see once all that bacteria and pus is removed from the mouth.
I hope this helps, let me know if you have any other questions.