Dogs have been known to eat some crazy items, both digestible and not, but there are human foods that can make dogs sick--even to the point of death.
Common Harmful Foods
Common household groceries that can cause digestive harm to canines include (but are not limited to) chocolate, onions, avocados, garlic, citrus fruits, potatoes, rhubarb, tomatoes, dairy items and nuts.
Less Common Harmful Foods
Less common foods or substances that are dangerous for dogs include drugs, alcohol, tobacco, baby food, some cat foods, yeast dough (which can expand in the intestine) and raw meats. In addition, there is always the danger of the dog swallowing a fruit pit or small bone that will become lodged in or puncture the intestines.
Different foods can cause different effects. For example, oxalates in potatoes and tomatoes can damage the digestive, nervous and urinary tracts. The sulfoxides and disulfides in garlic and onion can damage red blood cells and lead to anemia. Cat food, while usually not toxic, typically contains higher concentrations of fat and protein than is necessary for a dog; and dairy items can (much like in humans) cause bloating and diarrhea.
Symptoms of gastrointestinal distress or toxicity can include heavy panting, fever, diarrhea, muscle tremors, hyperactivity or seizures, among other reactions. If these symptoms are not immediately addressed, they can potentially lead to death.
Most people seem to know that chocolate is bad for dogs, but not all chocolate is created equal. Due to a substance called methylxanthine (also found in coffee and caffeine), the more concentrated the chocolate, the worse it can be. So, for example, baker's chocolate can be more lethal to a dog than white chocolate--but any level can be harmful to some degree.
If your dog shows signs of distress and the bag of Oreos is missing from the counter), call your veterinarian, an emergency clinic or the ASPCA Animal Poison ControlCenter at (888) 426-4435.