It is a really good idea not to get on a bb and tell people chocolate is like peanut butter for people - you will cause some dogs to die. Peanut hypersensitivity is relatively rare in humans. Human deaths from eating peanut butter are relatively rare.
A dog reacting badly to chocolate is NOT rare.
However, most vets will tell you that different types of chocolate have more (or less) theobromine, and that the amount of chocolate that will make a dog sick depends somewhat on the dog's weight, amount and type of chocolate, but also that there is an individual factor - some dogs are simply more sensitive than others.
Chocolate Toxicity in Dogs
(KEEP IN MIND THAT THE INFORMATION BELOW DOES NOT TAKE INTO ACCOUNT INDIVIDUAL SENSITIVITY - YOU MAY FOLLOW THESE RULES AND STILL KILL A DOG THAT HAPPENS TO SIMPLY BE MORE SENSITIVE TO THEOBROMINE)
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We've all heard it, "Don't give your dog chocolate it will kill him". We'll how true is it you're probably wondering. Do I have to rush him to an emergency vet if he ate one of my M&M's?
The truth is chocolate contains theobromine that is toxic to dogs in sufficient quantities. This is a xanthine compound in the same family of caffeine, and theophylline.
Toxic Levels
The good news is that it takes, on average, a fairly large amount of theobromine 100-150 mg/kg to cause a toxic reaction. Although there are variables to consider like the individual sensitivity, animal size and chocolate concentration.
On average,
Milk chocolate contains 44 mg of theobromine per oz.
Semisweet chocolate contains 150mg/oz.
Baker's chocolate 390mg/oz.
Using a dose of 100 mg/kg as the toxic dose it comes out roughly as:
1 ounce per 1 pound of body weight for Milk chocolate
1 ounce per 3 pounds of body weight for Semisweet chocolate
1 ounce per 9 pounds of body weight for Baker's chocolate.
So, for example, 2 oz. of Baker's chocolate can cause great risk to an 15 lb. dog. Yet, 2 oz. of Milk chocolate usually will only cause digestive problems.