Yes, buttercups are poisonous to humans and animals. Every species of the shiny yellow flower contains ranunculin, a natural chemical compound. When the plant is chewed, crushed, or bruised, this compound turns into a toxic, oily substance called protoanemonin. Why They Are Poisonous
The sap of the fresh plant acts as a strong blistering agent. Touching or eating buttercups can cause immediate irritation. Here is what happens when humans or animals interact with them:
Skin Contact: Touching crushed leaves can cause a skin rash, burning, itching, and blisters.
Eating Fresh Flowers or Leaves: Ingesting raw buttercups leads to severe mouth pain, blisters on the lips, a swollen face, vomiting, and bloody diarrhea. In rare and severe cases, it can cause dizziness, spasms, or even paralysis. Effects on Animals and Pets Buttercup Facts – SoftSchools.com