Most Helpful Girl
"The reason that chocolate is dangerous for dogs is due to one of the chemicals found within it – Theobromine. This is perfectly harmless to humans thanks to our digestive system; however it’s a different story for our four legged friends. Due to their inability to metabolize it fast enough, it can lead to a toxic build-up."2
Most Helpful Guy
"A small amount of chocolate will probably only give your dog an upset stomach with vomiting or diarrhea. With large amounts, theobromine can produce muscle tremors, seizures, an irregular heartbeat, internal bleeding or a heart attack. The onset of theobromine poisoning is usually marked by severe hyperactivity."
"The simple answer to your question is yes, chocolate is toxic; however, it is the ingredient theobromine that is contained in chocolate that is toxic to dogs. The toxicity is based on the amount of theobromine that the animal ingests when compared to the animal’s weight. It also matters what type of chocolate is ingested because the amount of theobromine in chocolate varies.
Any dose of theobromine over 45 mg/kg is potentially toxic and possibly lethal for dogs and should be treated aggressively by your veterinarian.
As an example, milk chocolate contains 44-64 mg theobromine per ounce of chocolate — so an average sized 80-pound Lab would need to ingest approximately 30-50 ounces of milk chocolate for a toxic dose. That is almost 3 pounds of milk chocolate! However, unsweetened baking chocolate contains 450 mg theobromine per ounce of chocolate. That means that same 80-pound Lab would be at a toxic dose after only 3.5 ounces!"