Substance Toxicity: 

If your pet has swallowed a possibly toxic substance (such as medication), please call 1-888-426-4435 for the Animal Poison Control Center. 

There are many household items that can be harmful to pets, including:

Antifreeze (radiator fluid)

Chocolate/Cocoa Products, grapes/raisins

Xylitol, an artificial sweetener (often found in chewing gum) 

Onions

Many kinds of indoor and outdoor plants (lilies are a common problem)

Wild/Outdoor mushrooms