Phenylethylamine
For information on amines in general please read the amines article.
Phenylethylamine is a biogenic amine which causes problems for some individuals including headaches.
For those sensitive to it, unpleasant symptoms arise when the level in their body exceeds the amount they can tolerate.
The symptoms can mimic those of allergy and lead to an incorrect diagnosis. Many 'chocolate' allergies have, on closer examination, turned out to be a sensitivity to phenylethylamine.
Further details can be found in
Change Your Diet and Change Your Life or
Food Intolerance and Migraine or