A not-insubstantial number of people have a casein allergy and may find themselves experiencing negative reactions both to casein-containing food products and to products such as nail polish that contain casein.http://www.wisegeek.com/what-is-casein.htmMany foods that are advertised as ???non-dairy??? or ???dairy-free??? still contain casein.Also a real life example here http://smallnotebook.org/