It's hard to eat enough cinnamon to affect most people, but some are extra-sensitive to it. Apparently there is a lot less in the stuff labeled cinnamon and sold to Americans, which is actually not cinnamon at all, but cassia, a whole other species. The difference was understood in Europe in ancient times, so there is no excuse. Sometimes you can find the real stuff in the US labeled "Ceylon cinnamon". It's a darker red than cassia, which is unapologetically brown.

Real cinnamon tastes more like red cinnamon candy than cassia does. Americans are always mystified as to why cinnamon candy doesn't taste anything like the cinnamon they put in their pies.