MSG is basically salt. so the headaches are most likely from dehydration. headaches induced by dehydration suuuuck. the most likely cause of hang over headaches is dehydration.
there are some pretty serious food allergies, just like there are anaphylactic reactions to medications, but whats bullshit is how large populations are slaves to the minority. Like when they won't serve peanuts on an airplane because one person claims to have a peanut allergy. there isn't airborne aerosolized penaut dust being emitted from the packages upon opening, and odds are that the previous crew/flight served peanuts on the same plane.