Chocolate: Although many people are aware that chocolate is toxic to cats and dogs, it's something that is commonly left on a table or counter top. Dogs may tear into a box of candy when the opportunity presents itself. In large quantities, chocolate causes coma and death. In lesser quantities, it will cause gastrointestinal problems including diarrhea. The degree of toxicity depends on the weight of the animal and the amount and type of chocolate consumed. Semi-sweet and bakers chocolate, for instance, contain more theobromine than regular chocolate, making them even more toxic to animals. Dogs love chocolate, so don't let taste be a test of what is healthy. Anything containing cocoa or chocolate should be avoided.
Grapes & Raisins: Though it isn't clear to scientists just what makes grapes and raisins toxic to both cats and dogs, even a relatively small amount can damage the kidneys. For this reason it's unwise to feed these to your pet, even if small amounts are tolerated. Also avoid giving bits of cookie or other foods that contain raisins.
Garlic, Onions, and Powders: Whether fresh, cooked or powdered, garlic and/or onions can be found among the ingredients of many prepared meals, including baby food. Garlic and onions can be toxic to cats and dogs because sulfoxides and disulfides found in them can damage red blood cells and lead to anemia. Onions are more problematic than garlic, but both should be avoided. If preparing a meal for yourself that will contain garlic or onions, consider preparing a side portion without these ingredients for your animals.
Mushrooms: Various species of mushrooms are toxic to dogs, and can cause shock and death. Effects will differ depending on the type ingested, but avoid feeding any type. Pluck "backyard mushrooms" that might sprout on the lawn, as these are known to be toxic to canines.
Tomatoes and tomato plants: Tomatoes of all kinds are toxic to cats, as are parts of the tomato plant. Ingesting as little as a cherry tomato can cause severe gastrointestinal upset.
Chicken Bones: Though not toxic, chicken bones can get stuck in the roof of the mouth, throat and intestines and should be avoided, according to many veterinarians. Splinters of chicken bones can also become lodged internally.
Other foods to avoid include raw potatoes, especially those containing green spots; turkey skin; nutmeg; and anything with caffeine, including tea. Alcohol and hops should also be avoided as alcohol poisoning can lead to coma and death. Do not allow your pet to rummage through the trash, as moldy food contains toxins that can cause serious illness.