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Hi, this is Dr. Hill. Hydrangea plants are a gastrointestinal irritant, so it is very common for vomiting to occur after ingestion. They do contain some cyanide, but have to ingest large amounts for it to be more than just an irritant. You can wait one hour after ingestion and if no additional vomiting, then offer a little bit of food or water. The irritation is typically short lived. If the kitty continues to vomit, then you definitely need to go to the emergency clinic. Feel free to ask additional questions. Take care.