Lily of the valley can kill dogs within hours by causing fatal heart arrhythmias.
All parts are toxic, including the water it has been standing in. The danger is similar to foxglove.
Lily of the valley contains over 30 different cardiac glycosides, including convallatoxin and convalloside. These inhibit the cardiac sodium-potassium pump and produce severe bradycardia, conduction blocks and ultimately ventricular fibrillation. Effects can be rapid and dramatic.
Sources
Even ingesting one or two leaves can be enough to cause clinical signs in a small dog. Treat any ingestion as an emergency.
Contact your veterinarian or an emergency animal poison hotline immediately. Do not induce vomiting unless instructed by a vet.
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