Poinsettias have a worse reputation than they deserve — they cause mild irritation but are rarely dangerous.
The milky sap is an irritant; ingestion typically causes drooling and mild stomach upset.
Poinsettias contain diterpene esters in the milky sap. These are mucous membrane irritants but are not systemically toxic in the amounts dogs typically ingest. The most common signs are drooling, lip-licking and a single bout of vomiting.
Quellen
Most exposures need only home monitoring; very large ingestions or symptoms beyond mild GI upset warrant a vet call.