The dangers of chocolate are widely known, but the composition of chocolate is less well known.
Cocoa beans are full of fat, which is mildly flavoured (think white chocolate). The chocolate flavour comes from the non-fat part of the cocoa bean and that's where the theobromine, the "bad" stuff is. In a given volume of chocolate the more fat there is, the less of the non-fat part there is.
Milk chocolate is so-called because it contains a high percentage of milk fat (yes, butter by another name). It contains varying amounts of the non-fat part of the cocoa bean--just enough for flavour but not enough for the characteristic bitter or "dark" flavour. Semi-sweet and dark chocolate has more of the non-fat part of the cocoa bean, which gives it a more intense and "darker" flavour. Baker's chocolate has still more of the non-fat part of the cocoa bean in it--which is why it is often used in recipes by melting it and adding butter. Baker's chocolate is a compact way to store the flavour of the chocolate, in other words.
Chocolate ice cream doesn't usually have a whole lot of actual chocolate in it--even less (in terms of volume) than a Hershey's plain chocolate bar. By law, ice cream must contain a certain percentage of butterfat in it--which reduces the amount of volume available for the non-fat part of chocolate.
The "problem" ingredient is theobromine. I hesitate to call it a problem because for humans, it's suspected to be the ingredient that makes us feel so nice after eating chocolate (that "chocolate bliss" feeling). For dogs, however, it is a problem because it's metabolized so slowly. Indications of an overdose are: increased nervousness or excitement, trembling, panting unconnected to high activity or heat, heart racing, seizures, coma. Some dogs (usually older ones) can have a heart attack triggered by the rapid heartbeat which forces their hearts to work harder.
The trickiest thing about the theobromine in chocolate is that dogs metabolize it very slowly--it takes about six days for it to leave the dog's body. So if you hand your dog a little chocolate on Monday, and then the dog (unbeknownst to you) gets some more chocolate from one of the kids on Tuesday, an unauthorized foray into the chocolate on Wednesday can cause problems.
The author M. Shirlely Chong is the list administrator for the K9-Cuisine List and can be reached via email at email@example.com.