Cats often vomit or develop diarrhea, when should we treat? The reason for the vomiting or diarrhea may be simple, such as a hairball, however the cause could be more serious. Whether the symptoms stop on their own, or whether your cat needs to see a vet, will depend on how he or she is in themselves and what the vomit or diarrhea looks like.To read more on this story, click here: Vomiting and diarrhea in cats