Dogs, for whatever reason, love to eat everything they can. They will even choose the kids toys over a fresh bowl of dog food if they had the choice. But just because they will eat pretty much anything in front of them, it does not mean their bodies can handle it.
Chocolate is a great treat for us, but can pose a great risk for dogs; it is essentially poisonous. What gives? Why are we allowed to indulge ourselves in this snack while dogs can only sit and watch? It comes down to the main ingredient in chocolate: cocoa.
Cocoa contains theobromine, an alkaloid similar to caffeine. Humans are able to metabolize this compound fairly quickly, but the process is much slower in dogs. In fact, it takes 17.5 hours to metabolize half of the theobromine in a dog's system. Since it is in the system for so long, it can have adverse effects. Consuming even a small amount can cause vomiting and diarrhea while larger doses can lead to high blood pressure, tremors, seizures, and death.
How much is too much?
There are many factors that determine what a lethal dose is for any particular dog. First off, milk chocolate is "safer" than dark or baker's chocolate because it contains less theobromine, while white chocolate has the least concentration of the compound. The Merck Veterinary Manual, a reference manual for veterinarians, states that one ounce of milk chocolate per pound can be lethal. The amount for dark chocolate is much lower.
The size of the dog also matters, with the smaller ones being at a greater risk than their large counterparts. According to a vet's quote in this WebMD article:
20 ounces of milk chocolate, 10 ounces of semi-sweet chocolate, and just 2.25 ounces of baking chocolate could potentially kill a 22-pound dog." For comparison, 20 ounces of milk chocolate is about 13 hershey bars.
While we're on the subject
Dogs are not the only animal that can be harmed from theobromine. Cats can die from smaller doses, but they are pickier eaters than dogs and do not have a sweet tooth for chocolate. Rats and mice (and even humans) can also die from theobromine poisoning, all requiring around the same toxicity level.
Long story short, don't go feeding your dog chocolate. While a small amount isn't necessarily lethal, theobromine's side effects are not worth the risk.