Some foods commonly enjoyed by humans are dangerous to dogs:

Dogs love the flavor of chocolate, but chocolate in sufficient doses is lethally toxic to dogs (and horses and possibly cats). Chocolate contains theobromine, a chemical stimulant that, together with caffeine and theophylline, belongs to the group of methylxanthine alkaloids. Dogs are unable to metabolize theobromine effectively. If they eat chocolate, the theobromine can remain in their bloodstreams for up to 20 hours, and these animals may experience fast heart rate, hallucinations, severe diarrhea, epileptic seizures, heart attacks, internal bleeding, and eventually death. A chocolate bar can be sufficient to make a small dog extremely ill or even kill it. Approximately thirty grams of baking chocolate per kilogram (1/2 ounce per pound) of body weight is enough to be poisonous. In case of accidental intake of chocolate by especially a smaller dog, contact a veterinarian or animal poison control immediately; it is commonly recommended to induce vomiting within two hours of ingestion. Large breeds are less susceptible to chocolate poisoning, but still are far less tolerant of the substance than humans are.
Note:Carob treats are often available as dog treats; these are unrelated to chocolate and are safe.

It has recently been confirmed that grapes and raisins can cause acute kidney failure in dogs (see also grape and raisin toxicity in dogs). The exact mechanism is not known, nor is there any means to determine the susceptibility of an individual dog. While as little as one raisin can be toxic to a susceptible ten pound dog, some other dogs have eaten as much as a pound of grapes or raisins at a time without ill effects. The affected dog usually vomits a few hours after consumption and begins showing signs of renal failure three to five days later.

Onions, and to a significantly lesser extent garlic, contain thiosulfate which causes hemolytic anemia in dogs (and cats). Thiosulfate levels are not affected by cooking or processing. Small puppies have died of hemolytic anemia after being fed baby food containing onion powder. Occasional exposure to small amounts is usually not a problem, but continuous exposure to even small amounts can be a serious threat. Garlic is also known to cause diarrhea and vomiting.

Macadamia nuts can cause stiffness, tremors, hyperthermia, and abdominal pain. The exact mechanism is not known. Most dogs recover with supportive care when the source of exposure is removed.

Alcoholic beverages pose much the same temptation and hazard to dogs as to humans. A drunk dog displays behavior analogous to that of an intoxicated person. (However, beer presents another problem; see below.)
Hops, a plant used in making beer, can cause malignant hyperthermia in dogs, usually with fatal results. Certain breeds, such as Greyhounds, seem particularly sensitive to hop toxicity, but hops should be kept away from all dogs. Even small amounts of hops can trigger a potentially deadly reaction, even if the hops are "spent" after use in brewing.

Xylitol is a sugar substitute used in chewing gum, chewable vitamins, candy, toothpaste, and other products. Although empirical studies indicate xylitol may be safe for dogs, there have been cases of foods, candies and gums containing xylitol causing toxic or even fatal liver damage in dogs and should be avoided.

Some dogs have food allergies just as humans do; this is particular to the individual dog and not characteristic of the species as a whole. An example is a dog becoming physically ill from salmon; many humans likewise have seafood allergies.

If dogs eat the pits of fruits such as peaches and apricots, they can get cyanide poisoning due to cyanogenic glycosides.