Giardia and C. difficile infections certainly can cause diarrhea and vomiting however they are not the most common causes. Metronidazole has been prescribed as a general anti-diarrhea medication however I would be concerned that your cat is vomiting as well. When I see older cats with GI signs I become concerned with IBD (inflammatory bowel disease) as well as unfortunately cancer. A series of radiographs and blood tests should be performed.
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