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Thank you for your question. My name is***** I have been a small animal veterinarian for 13 years and I'd like to help with your questions.
Biotin is a water soluble vitamin so the body take any excess amounts and excretes it in the urine. It should not cause any serious problems. It could cause mild to moderate stomach upset and make the urine a dark yellow color. Try to get him to drink a lot... you can offer diluted tuna juice. If the stomach upset is severe and includes vomiting or severe diarrhea then I would take him to a veterinarian for some anti nausea medications.
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