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Thank you for your question. I am a veterinarian and will do my best to help. I answer questions for this website. Where did you see that fennel is toxic?
I have not heard that fennel is toxic, so I checked on VIN (veterinary information network) with a veterinarian who works with ASPCA poison control, and she says it is not considered a toxic plant; it does have some medicinal properties (as a diuretic, or "water pill"). Small amounts should not pose a problem, but large amounts may lead to increased drinking and urinating.
Let me know if this helps and what else I can answer or research for you. Rebecca