Rhubarb leaves contain oxalates that damage the kidneys and irritate the mouth.
The stems are less toxic than the leaves but still irritating; the leaves are the dangerous part.
Rhubarb leaves are rich in soluble calcium oxalate crystals. These bind to calcium in the blood, causing hypocalcemia, and precipitate in the kidney tubules causing acute kidney injury. The crystals also cause immediate mouth and throat pain on contact.
Sources
Even a single mouthful of leaves can cause symptoms in small dogs. Cooked stems in small amounts are less risky but still best avoided.
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