Lily of the Valley is a highly very highly poisonous plant that contains poisons called cardiac glycosides.
The Lily of the Valley plant becomes poisonous to humans or dogs if ingested when they eat part of the plants which can happen with babies and small children who put things in their mouth without knowing any better.
So if you have dogs or small children or babies you should never have a Lily of the Valley Plant in your house or yard where babies or small children or even dogs can try to eat part of the plant.
The cardiac glycosides that Lilly of the Valley contains will cause symptoms similar to Foxglove (digitalis) ingestion when ingested and can kill a human or dog if not treated.