A. It should not be a major concern, as he probably ingested a minimum dosage which may have not even been absorbed.
The overdosage of gentamicin could cause kidney damage, hearing loss, or vestibular (balance) disorders. May also see facial swelling, pain at injection site, allergic reactions, vomiting, diarrhea, blood disorders, or liver disorders.
Monitor him for the next 24-48hours and if concern for any signs take him to your veterinarian for a check up.
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