Most true ferns, such as Boston fern, maidenhair fern, button fern, and sword fern, are not toxic to cats. If a cat eats a small amount of these true ferns, they are unlikely to be harmed, though large quantities might cause mild stomach upset like vomiting or diarrhea. However, some plants that look like ferns or have "fern" in their name, such as asparagus fern, foxtail fern, and lace fern, are toxic to cats and can cause symptoms including vomiting, diarrhea, stomach pain, drooling, and skin irritation. It is best to keep toxic fern-like plants out of reach or avoid them in homes with cats. True ferns are generally safe but discouraging cats from eating any plants is wise for their overall health.