I used to be sensitive to MSG. I first noticed it before it was something that everyone had heard of, before I had heard of it. I was always a bit sceptical about the hype, because the so-called MSG headache isn't really a headache. It's a weird pulling of the muscles around the edge of the face and hairline and can cover the entire scalp. I wouldn't call it painful, just weird. So, I strictly avoided MSG for decades. I actually found Chinese restos to be the surest place to avoid MSG. Tell them you don't want it, they don't put it in the food. They don't try to sneak a little bit in that you "probably won't notice".
I don't seem to have the sensitivity any more. I discovered that when the local Chinese buffet quit having a "No MSG table". They simply cut down in general on the MSG. I gave it a try and had no reaction.
I still avoid it, but not with any vehemence. Too much and it can give a weird, off flavour. I find the weird flavour similar to the weird flavour of potassium chloride, which my mother used for a while as a table salt substitute.
It was never an allergy. There is no histamine reaction. It's a food sensitivity.
May you live as long as you wish and love as long as you live.
Robert A. Heinlein