It depends on the amount and size of the breed. Watch for shaking, excessive panting, lethargy, if in doubt call a vet asap. Ive seen deaths of Dogs with theobromine poisoning. It effects their blood pressure breathing etc... Call a vet if you need further help While the pathetic begging look that goes across the face of a dog wanting chocolate can weaken the most stoic dog owner, stay firm. Do not give in. Ever. SYMPTOMS OF CHOCOLATE POISONING You can recognize that your dog has eaten a toxic dose of chocolate from the symptoms. Within the first few hours, the evidence includes vomiting, diarrhea or hyperactivity. As time passes and there's increased absorption of the toxic substance, you'll see an increase in the dog's heart rate, which can cause arrhythmia, restlessness, hyperactivity, muscle twitching, increased urination or excessive panting.