Cacao beans contain about 1.2% theobromine by weight, while processed chocolate, in general, has smaller amounts. The amount found in highly refined chocolate candies (typically 1.4–2.1 g/kg or 40–60 mg/oz) is much lower than that of dark chocolate or unsweetened baker's chocolate (> 14 g/kg or > 400 mg/oz).
A typical 20 kg (44 lb) dog will normally experience intestinal distress after eating less than 240 g (8.5 oz) of dark chocolate, but won't necessarily experience bradycardia or tachyarrhythmia unless it eats at least 0.5 kg (1.1 lb) of milk chocolate. According to the Merck Veterinary Manual, baker's chocolate of approximately 1.3 g/kg (0.02 oz/lb) of a dog's body weight is sufficient to cause symptoms of toxicity. For example, a typical 25-gram (0.88 oz) baker's chocolate bar would be enough to produce symptoms in a 20-kilogram (44 lb) dog.
Reference:
http://en.wikipedia.org/wiki/Theobromine_poisoningEight ounces of Hershey dark chocolate makes me feel sick. I don't know if that five-pound bar would kill me. Let us do the math.
Let's say the Hershey has 2 g/kg = 2,000 mg/kg. A 2.6 kg bar would contain 2,000 mg/kg x 2.6 kg = 5,200 mg. I Weigh 91 kg. The five pound bar would give me 5,200 mg/91 kg = 57 mg/kg poison. Not a lethal level (1,000 mg/kg) but, for me, a throw-up level. Of course, throwing up would eliminate the problem, wouldn't it?