Losing a baby tooth should not cause her to lose her appetite so I would think something else is going on. My concern is more with your vet and his lack of knowledge regarding the treatment of a dog with double teeth. While it is common for smaller breeds to retain teeth it is not acceptable for them to be left in the mouth for 5 years. Retained teeth can cause periodontal disease and that can lead to many other health issues. Is your little one spayed? Normally retained teeth are removed during the spay/neuter.
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