The simple answer to your first question is "Yes, it is safe".
Here is the detailed answer:
Monosodium glutamate is a commonly used flavour enhancer. Basically it is a sodium salt of the normally present amino acid - glutamate - found naturally in our bodies and in many protein-containing foods such as cheese, milk, meat, peas, and mushrooms. Part of the flavour-enhancing effect of tomatoes is attributed to their high content of glutamate. In its salt form MSG appears as a crystalline white powder. When dissolved in water or saliva it rapidly dissociates into free sodium and glutamate ions.
Did you know that the 'taste' induced by MSG is called
"Umami"?
There are allegations that MSG is responsible for many adverse health conditions, including MSG intolerance, characterized by symptoms such as headache, nausea, digestive upsets, drowsiness, palpitations, bronchosapsm, anaphylactic shock etc. "Chinese restaurant syndrome" is often used as an example of the symptoms purported to be caused by MSG.
Neurotoxicity and excitotoxicity were also attributed to MSG. It was also demonstrated in newborn laboratory mice that MSG can induce adult obesity.
However, lets look at the scientific evidence. I will refer to these publications related to MSG available on PubMed:
If you go through these resources, you will realize that none of the above allegations against MSG have been demonstrated in scientific studies.
Some important quotes:
L-Glutamic acid and its ammonium, calcium, monosodium and potassium salts were evaluated by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in 1988.
Human infants metabolized glutamate similarly to adults. Conventional toxicity studies using dietary administration of MSG in several species did not reveal any specific toxic or carcinogenic effects nor were there any adverse outcomes in reproduction and teratology studies.
Because human studies failed to confirm an involvement of MSG in "Chinese Restaurant Syndrome" or other idiosyncratic intolerance, the JECFA allocated an "acceptable daily intake (ADI) not specified" to glutamic acid and its salts. No additional risk to infants was indicated.
The conclusions of a subsequent review by the Federation of American Societies for Experimental Biology (FASEB) and the Federal Drug Administration (FDA) did not discount the existence of a sensitive subpopulation but otherwise concurred with the safety evaluation of JECFA and the SCF.
Results of surveys and of clinical challenges with MSG in the general population reveal no evidence of untoward effects.
The results suggest that large doses of MSG given without food may elicit more symptoms than a placebo in individuals who believe that they react adversely to MSG. However, the frequency of the responses was low and the responses reported were inconsistent and were not reproducible. The responses were not observed when MSG was given with food.
Hope this information answers your queries.
CLINICAL SUBJECTS 
CLINICAL SUBJECTS
