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Does it happen to be Nutro Natural Choice?
If it is, this may help:
Consumer complaints about Nutro Pet Foods
Nutro has been blamed for the deaths and illnesses of many dogs.
If he isn't being fed this, it is always a good idea to bile acids test a Yorkie to look for liver problems and rule them out because the breed is so prone to liver shunt.
Was it metronidazole that the vet put him on?
Did the vet offer to put him on it for a longer period of time because it takes some dogs longer to recovery?