Hostas are toxic to dogs due to containing saponins, which are bitter- tasting phytochemicals that cause gastrointestinal distress. Common symptoms in dogs who ingest hostas include vomiting, diarrhea, lethargy, depression, and loss of appetite. While mild cases may resolve without veterinary treatment, more severe or persistent symptoms should be evaluated by a vet. It is advised to keep dogs away from hosta plants to avoid these adverse effects.