MSG is accused of being an excitotoxin by some. While double-blind peer-reviewed research is very useful, it is never definitive. For instance, a number of studies of Stevia extracts concluded that they are safe while six of them found concerns/evidence regarding DNA damage. Unfortunately, there is a big conflict of interest in the studies world due to the fact that the best sources of funding tend to be corporations trying to sell products and government which is often in bed with corporations. This can result not only in purposefully misleading research but, probably more often, a simple lack of adequate research. That's the charge aimed at acesulfame-K, for instance, by quite a few professionals (see the CSPI article I linked to). It's also a problem with the Stevia extract rebaudioside-A ("Rebiana"), which, according to UCLA toxicologists, has not been subjected to true lifetime studies in both rats and mice.