I showed the Sykes the chocolate toxicity calculator* which told us that 157 grams of milk chocolate in a 10 kg beagle translated into 35 mg/kg of the active toxic ingredient, which was in the "mildly toxic" range, likely to produce vomiting, diarrhea, shaking and an increased heart rate. Fortunately we had only seen the first symptom, in part because the Sykes knew their Elwood and had rushed him in immediately after the futile "Elwood! Drop it!!" so that we could induce vomiting and get as much out of him as possible. As an aside, I want you to know that although veterinary clinics can be awash in a potpourri of vile smelling substances, chocolate vomit holds a special place near the apex of the devil's perfumerie. I mention this only so that you know that the veterinary staff also suffers when you allow your dog access to chocolate. But I digress.
So chocolate is poisonous to dogs, this much most of you know. But do you know why it is poisonous? The aforementioned active toxic ingredient is theobromine, which is in the same methylxanthine class of stimulants as caffeine. What makes dogs different is that they metabolize it much more slowly than humans. Cats do too, but they are almost never interested in eating enough chocolate for it to matter as they can't appreciate the sweetness. Because it is a stimulant, at a high dose it can cause severe heart rhythm disturbances and potentially fatal seizures. At about 200 mg/kg of theobromine 50% of untreated dogs will die. Theobromine content varies between types of chocolate, with milk chocolate having the least and baker's dark chocolate having the most. As a general rule of thumb, 28 grams (1 oz) of milk chocolate contains approximately 60 milligrams of theobromine, while the same amount of dark chocolate contains about 200 milligrams and baker's 400 mg.
It may be of interest to note that a recent study of 230 vet clinics in England indicated that the risk of chocolate poisoning was four times higher at Christmas than any other time of year except Easter, when it was two times higher. Curiously, there was no increased risk on Valentine's Day or Halloween (although, mind you, the latter is a much smaller chocolate event in the UK than over here and the former usually involves more expensive closely guarded chocolates).
Incidentally, it is theoretically poisonous in humans as well, although we are much less sensitive. A person my size would have to eat about 4.5 kg of baker's dark chocolate, or an impressive 32.5 kg of milk chocolate to be at significant risk of Death By Chocolate. I would imagine that an array of increasingly distressing feelings would precede the fatal overdose and prevent you from getting to that point. But imagine the obituary...
* Here is the play at home version: http://veterinaryclinic.com/chocolate/calc.html
I caution you to please please call your clinic or emergency line regardless though, rather than relying on the online calculator. If you are in a remote location or otherwise unable to reach a clinic you can try to induce induce vomiting by administering 3% hydrogen peroxide at the rate of 1 teaspoon per ten pounds. You will probably need a syringe or a turkey baster to get the poor guy to take it. But he will regret nothing.