The peace lily is a popular indoor tropical plant that does well in low light, which is why it's also called a "closet plant." With its long, spear-like leaves and brilliant white sail-shaped flowers, it looks innocent, but it can be a hazard to your dog or your children.
The plant is not poisonous in the scientific or legal sense, but all parts of it -- leaves, flowers, stems and roots -- contain crystals of a chemical compound called calcium oxalate. These crystals are one way the plant defends itself against being eaten, since they cause unpleasant physical results when ingested. Anyone who chews on the peace lily, from caterpillars to dogs to babies, gets an immediate and lasting lesson not to do it again.
Calcium oxalate is not a toxin, but rather a mechanical irritant. It causes burning pain and swelling of the mucous membranes of the mouth, throat and digestive tract -- if it gets that far. It will make a dog (or a human) very uncomfortable, but is unlikely to kill or cause permanent injury. If you notice your dog drooling, whining and pawing at his mouth, check your houseplants for chewed or broken leaves. He may whine or whimper, but be unable to bark. He may vomit, and he probably will not want to eat.
Clean any plant residue out of your dog's mouth and irrigate his mouth with cool water to dilute and wash away the crystal-containing juice; do this gently and hold his mouth downward so that he doesn’t choke or inhale water. If he can swallow, offer him ice chips or even ice cream to lick to ease the pain. If he continues in distress or has trouble breathing (because of swelling in the throat), call a vet and take him in for emergency care.
Many common tropical houseplants use this defense mechanism, including philodendron, pothos ivy, anthuriums, caladiums and Dieffenbachia, which is also known as "dumbcane" or "mother-in-law plant" because eating it can cause temporary speechlessness. If any toxic plants in your house, yard, garden or anywhere your dog goes, elevate them, fence them off or otherwise put them beyond his reach.