Many human foods are toxic to dogs, including chocolate, grapes and onions. Many candies and other sweet foods are dangerous because they contain xylitol, an artificial sweetener that is highly toxic to dogs.
Veterinarians do not fully understand why grapes and raisins are toxic to dogs, and some dogs do seem to be able to eat them safely. However, it is best to avoid feeding them because the poisoning is unpredictable and even small amounts have made dogs very sick.
The pits of various fruits, including cherries, peaches and apricots, are toxic, so these fruits should only be fed after removing the pit. Avocados are mildly toxic and can cause digestive upset and diarrhea. Apple seeds and cores are toxic, although apple slices without these parts are safe. Ripe tomatoes and potatoes are generally safe, but when they are green they are toxic.
Macadamia nuts and walnuts are also toxic to dogs even in small amounts. Macadamia nut poisoning is not usually fatal but it can be painful and may require expensive treatment.
Onions and garlic damage the dog's red blood cells, so symptoms may not be apparent immediately. Dogs are generally safe when eating raw garlic or onions because they do not eat enough to reach toxicity, but eating powdered or dehydrated versions of these foods often results in poisoning.