Mono sodium glutamate, or E621 as it is sometimes listed on food labels, is best avoided. Even though it is regarded as safe in small doses(up to 3 grams considered safe) it can have some rather unpleasant side effects including Headache, nausea, vomiting and even breathing difficulties. As such it is best given a large body swerve and should never be included in a dish if the person it is served to has any history of asthma.