An Excitotoxin is defined by the Medical Dictionary as "A neurotransmitter (e.g., glutamate or aspartate) that can cause brain cell injury or death if its action is unabated. Brain damage is mediated by excitotoxins during prolonged seizure activity and stroke." 
There are many kinds of Excitotoxins, but perhaps the most well known is Monosodium Glutamate, an artificial food addictive which tricks the human sense of taste. Other glutamates have also become common in processed foods. As Monosodium Glutamate became better known, these others became popular, including whey protein and whey protein isolate, yeast extract, corn oil, soy products, and more.
Glutamates and other excitotoxins are believed by some to cause brain damage  such as ADHD and Alzheimer's Disease. Extended, repetitive exposure is believed to have a cumulative effect, making the risk of noticeable damage ever higher.
- "Excitotoxin." Medical Dictionary. 2009. Farlex and Partners 3 Dec. 2015 <http://medical-dictionary.thefreedictionary.com/excitotoxin>
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