My DS (4.5) has often shown some very bad reactions to chocolate. The worst tantrums he has had have been related to chocolate.
Today's example: he requested chocolate milk today, I decided it was okay. He had it about 2 hours after his last meal, so it was probably on an empty stomach. He became very mean and nasty, started to tell us we couldn't go inside or something else irrational and began to throw a fit. It took him about 20 minutes to come out of it, and only when I managed to convince him to have 8 oz or so of kefir.
Last week, I had taken him to the eye doctor, and then gave him ice cream after as a treat. He had a chocolate dipped cone. He went beserk at home (again, mean, then tantrumy behavior).
He used to drink chocolate milk in the mornings, and almost without fail, he would have a bad tantrum on those mornings. I eventually phased it out (he had been weaning from breastmilk, so had to be offered something equally attractive!:)), and the tantrums disappeared.
I figured rather than the chocolate itself, it was probably the sugar. And, even in the cases I have listed here, it could well have been sugar, as he was taking it on an empty stomach (though with protein). So, maybe he was just coming down from the sugar high. But still, he just doesn't have these types of reactions when he eats other kinds of sugar (which we limit, but occasionally comes to him from preschool, parades, what-have-you). I talked to him about it after, asked him what he was feeling, what was driving him, and he had no idea. I could tell he was totally out of control and irrational.
So, just thought I would run it by the MDC community!