Highly toxic — can be fatal
Yes — extremely. Oleander is one of the most poisonous garden plants for pets (and people). Every part affects the heart and even small amounts can be fatal.
Oleander (Nerium oleander) contains powerful cardiac glycosides that disrupt the heart’s rhythm — chemically similar to the heart drug digoxin. All parts are toxic, including dried leaves and even water the cut stems have sat in, and the toxin survives drying and burning. Ingestion can cause severe vomiting, dangerous changes in heart rate, tremors and death. It is a common ornamental hedge in warm climates around the Mediterranean, the southern US and Australia, which makes it a real-world risk.
Both dogs and cats, and grazing animals, are gravely affected. There is no safe amount; even a few leaves can be lethal to a small pet.
This is a life-threatening emergency — call your vet or a poison helpline immediately if you suspect contact, and do not wait for symptoms. Never keep oleander where pets roam, and be careful with clippings and bonfire smoke.
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General information, not veterinary advice. If your pet has eaten a toxic plant or shows symptoms, contact your vet or a pet poison helpline immediately. Full disclaimer.