A study revealed that only one out of every 500 people who thought they were allergic to chocolate actually tested positive. Because people complain of headaches and migraines after eating chocolate, they assume it is an allergic reaction. These reactions may be due to a food intolerance or sensitivity not an actual food allergy. It is also possible that adverse reactions could be caused by ingredients added to chocolate during processing, such as corn syrup, milk, soy beans, lecithin, gluten, and nuts.