isn't that an antibiotic? i looked it up and game up with a metronidazole thats why i'm asking. it says side effects are nausea, diarrhea, and/or metallic taste in the mouth. if it's an antibiotic i wouldn't worry too much, personally. i'd just watch for weird behavior. it also says High doses and/or long-term systemic treatment with metronidazole is associated with the development of black hairy tongue, leukopenia, neutropenia, increased risk of peripheral neuropathy and/or CNS toxicity.[1]
modified to add a pic of "black hairy tongue" because the mental pic of a chihuahua running around with it about killed me!