The way I understanding is that MSG is toxic in a similar way to alcohol. In small doses most people can tolerate it ,some more than others . At the time I had my "overdoses" I was eating a lot of sushi and stir fry , both with abundant use of soy sauce, that usually has MSG in it
It is also present in tomatoes and I eat a lot of them..
MSG (E621) is commonly listed as Hydrolized Proteins or Natural flavorings.
Franco