Why is chocolate toxic to dogs?

Chocolate contains theobromine and caffeine, both methylxanthines that stimulate the nervous system and heart. Humans metabolize theobromine relatively quickly; dogs clear it far more slowly, so it accumulates to toxic levels. Symptoms typically appear within a few hours and include vomiting, diarrhea, restlessness, rapid breathing, increased heart rate and thirst, and in severe cases tremors, seizures, and cardiac arrhythmia. The danger depends on three variables: the type of chocolate, the amount eaten, and the dog's body weight.

Type matters enormously because theobromine concentration varies by an order of magnitude. Baking chocolate and cocoa powder are the most concentrated, followed by dark chocolate, then semisweet, with milk chocolate substantially lower and white chocolate containing essentially none. A small dog eating a modest quantity of dark chocolate can be in serious trouble while a large dog eating the same weight of milk chocolate may only have an upset stomach. Veterinary chocolate toxicity calculators exist and are worth using rather than guessing.

If a dog eats chocolate, call a veterinarian or a pet poison hotline with the dog's weight, the type of chocolate, and the estimated amount, and do so promptly rather than waiting for symptoms, since treatment is far more effective before absorption is complete. Do not induce vomiting without professional direction. Several other common human foods are dangerous to dogs and are worth knowing together: xylitol, a sweetener in sugar-free gum, candy, and some peanut butters, causes a rapid and potentially fatal drop in blood sugar and is arguably a greater emergency than chocolate; grapes and raisins can cause kidney failure through a mechanism only recently identified; and onions, garlic, macadamia nuts, and alcohol are all toxic.