If a dog has eaten onions or garlic, he may show signs of toxicity 3-5 days afterwards, including fatigue, weakness or dark- or orange-colored urine. A dog displaying these symptoms should be taken to a veterinarian immediately.
According to the ASPCA, members of the onion family, including shallots, garlic, onions and scallions, contain sulfoxides and disulfides that, if consumed in sufficient quantity, can damage a dog's red blood cells and cause anemia. Ounce for ounce, garlic is more toxic than onions. Usually, dogs do not eat enough raw onions or garlic to cause serious health problems, but even dehydrated onions, garlic powder and onion soup mix may be harmful. A veterinarian can diagnose the dog's toxicity and prescribe medicine. In severe cases, a blood transfusion may be required.
Peteducation lists a number of other foods which should not be given to dogs. Poultry bones can obstruct or pierce the digestive system. Grapes and raisins contain a toxin which can damage the kidneys. Milk can cause diarrhea. Moldy food or garbage can have multiple toxins, causing vomiting and diarrhea. Even table scraps should be avoided, as they do not contain a balanced nutritional diet for a dog. They should not make up more than 10% of the dog's total food intake.