"Though it's certain that dogs and chocolate don't mix, different dogs react differently to the methylxanthines. And reactions can vary according to the age, size and overall health of the dog. The smaller the dog the smaller the dose needed to produce an effect. And of course, if your dog's health is already weakened by other medical conditions, he's more susceptible. The same holds for older dogs.. Theobromine is present in differing amounts in different kinds of chocolate:
White chocolate 1mg/oz
Hot chocolate 12 mg /oz
Milk chocolate 44-66 mg/oz
Semi-sweet chocolate 260 mg/oz
Dark chocolate 450 mg/oz (wow!)
Baking/bitter chocolate or cocoa powder varies as much as 150-600 mg/oz.
How much chocolate can a dog eat and survive?
Under 200 mg theobromine per kg body weight (91mg per lb) has not been observed to be fatal. A potentially lethal dose in a 16 pound. dog is only one pound (16oz) of milk chocolate. Only two ounces of baking chocolate can cause serious problems in a 10-pound dog. People stop eating chocolate before getting to toxic levels, but dogs don't!"
http://www.dibdabdoo.com/pets/chocanddog.htm
This is from an article I found... obviously, baking and dark chocolate are horribe, but milk chocolate, they can tolerate a little. I had a similar incident with Duke rummaging through my purse and finding an m&m cookie. We got it away from him, but not without a fight! I have to be careful where I set my purse because he thinks he is allowed to snoop around in it.