Chocolate has caffeine, but not enough that it should cause effects with breathing.

It does have a lot of butterfat, which can increase mucus production - and that can definiately affect breathing in someone who's got issues to being with. if you are allergic to it yes. If anything, it might improve your breathing as chocolate has caffiene (which is similiar to theophylline, a bronchial dilator). I used to suffer from asthma, and a morning cup of coffee helped me to breathe more easily. My husband struggled with emphysema and he used to eat chocolate without any problem regarding his breathing.

But the chocolate would sometimes give him pains in his chest because of acid reflux....which in turn would cause tension of the body making breathing a little more difficult.