No, hostas are not lethal to cats. However, they can cause stomach upset if eaten in large quantities. The leaves of the plant contain saponins, which are toxic to animals. Saponins are a type of glycoside that can be found in a variety of plants. When ingested, they can cause vomiting, diarrhea, and stomach pain. While most animals will avoid eating plants that contain saponins, some may be curious enough to take a bite. If your cat eats hostas, watch for signs of gastrointestinal distress and contact your veterinarian if necessary.