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How to clean a ferret's teeth
Get a small toothbrush and pet toothpaste.
Your local pet supply store will carry toothbrushes designed for kittens and puppies that are small enough for a ferret. It'll also have pet toothpaste, which is safe for a pet to swallow.
Never use human toothpaste to brush a pet's teeth. Human toothpaste contains too much fluoride to be safe for ferrets.