Having a bad reaction to MSG could also mean you are deficient in vitamin B6
Here's some details on MSG from the FDA:
In otherwise healthy MSG-intolerant people, the MSG symptom complex tends to occur within one hour after eating 3 grams or more of MSG on an empty stomach or without other food.
A typical serving of glutamate-treated food contains less than 0.5 grams of MSG.
A reaction is most likely if the MSG is eaten in a large quantity or in a liquid, such as a clear soup.
Severe, poorly controlled asthma may be a predisposing medical condition for MSG symptom complex.
Under current FDA regulations, when MSG is added to a food, it must be identified as "monosodium glutamate" in the label's ingredient list. Each ingredient used to make a food must be declared by its name in this list.
MSG is the sodium salt of the amino acid glutamic acid and a form of glutamate. It is sold as a fine white crystal substance, similar in appearance to salt or sugar. It does not have a distinct taste of its own, and how it adds flavor to other foods is not fully understood. Many scientists believe that MSG stimulates glutamate receptors in the tongue to augment meat-like flavors.
Asians originally used a seaweed broth to obtain the flavor-enhancing effects of MSG, but today MSG is made by a fermenting process using starch, sugar beets, sugar cane, or molasses.
Glutamate itself is in many living things: It is found naturally in our bodies and in protein-containing foods, such as cheese, milk, meat, peas, and mushrooms.
Some glutamate is in foods in a "free" form. It is only in this free form that glutamate can enhance a food's flavor. Part of the flavor-enhancing effect of tomatoes, certain cheeses, and fermented or hydrolyzed protein products (such as soy sauce) is due to the presence of free glutamate.
Hydrolyzed proteins, or protein hydrolysates, are acid-treated or enzymatically treated proteins from certain foods. They contain salts of free amino acids, such as glutamate, at levels of 5 to 20 percent. Hydrolyzed proteins are used in the same manner as MSG in many foods, such as canned vegetables, soups, and processed meats.
An unknown percentage of the population may react to MSG and develop MSG symptom complex, a condition characterized by one or more of the following symptoms:
The full FDA article can be found at http://vm.cfsan.fda.gov/~lrd/msg.html