Monosodium glutamate (MSG) is a seasoning mainly used in Chinese food, canned vegetables, soups and processed meats. The FDA states that this ingredient is "generally recognized as safe", but many still say it is unhealthy. Some things considered as MSG symptoms include:
Facial pressure or tightness
Numbness, tingling or burning in the face, neck and other areas
Rapid, fluttering heartbeats (heart palpitations)
Despite some claiming these symptoms, researchers has concluded that there is no link between MSG and these symptoms according to Mayo Clinic, but people still attempt to avoid MSG like the plague. Many restaurants include signs like these to attract non-ethnic customers.
The Fear of MSG started when a scientist made the term "Chinese Restaurant Syndrome" in a letter to The New England Journal of Medicine. Robert Ho Man Kwok claimed he experienced heart palpitations, weakness, and strange sensations of numbness after eating large amounts of Chinese food, and his colleagues decided that MSG must be the culprit.
The only situation where MSG can be bad for you, is if you consume a large amount of it on an empty stomach. Keep in mind, just like salt or any other seasoning, too much of anything can give you a bad reaction.