What is MSG and is it bad for you?

MSG or monosodium glutamate is a flavor enhancer added to Chinese food, canned foods and processed meats. Some people may experience an allergic reaction after eating foods containing MSG. Symptoms include headache, sweating, numbness and tingling in the face and neck, rapid heartbeat, etc. People with high blood pressure or heart problems should limit or avoid MSG because it contains sodium.
Reference: http://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/expert-answers/monosodium-glutamate/faq-20058196