Some Vets might keep this a secret but Cats get a kitty cold flu thing sometimes and what works for us is one of these.
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You stick the end in the side between the teeth and water them often. Then go get some of this in the little plastic container, best chance they eat.

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You can still take them to the Vet if you want but I think you should do what I said also.
![Drink up [B-o]](./images/smilies/drinkup.gif)