From Wikipedia's Monosodium Glutamate:
It is a popular belief that MSG can cause headaches and other feelings of discomfort, known as "Chinese restaurant syndrome", but blinded studies show no such effects when MSG is combined with food in normal concentrations, and are inconclusive when MSG is added to broth in large concentrations.
... In a 1993 study, 71 fasting participants were given 5 g of MSG and then a standard breakfast. One reaction (to the placebo, in a self-identified MSG-sensitive individual) occurred. A 2000 study tested the reaction of 130 subjects with a reported sensitivity to MSG. Multiple trials were performed, with subjects exhibiting at least two symptoms continuing. Two people out of the 130 responded to all four challenges. Because of the low prevalence, the researchers concluded that a response to MSG was not reproducible.
All forms of glutamate/glutamic acid/MSG are exactly the same from a chemical standpoint. The human body cannot tell the difference between the amino acid, glutamic acid/ the salt form of that acid, monosodium glutamate; or the ionic form, glutamate. You might want to invent some difference between the three forms, to make some case that MSG is more dangerous, but that would require a complete rewriting of what we know about basic chemistry. And you’d win a Nobel Prize for doing so.
See also: Food.