Excitotoxicity. I learned about this from Dr Drew while listening to The Adam Carolla Podcast.
Wikipedia defines it as:
Excitotoxicity is the pathological process by which nerve cells are damaged and killed by glutamate and similar substances. This occurs when receptors for the excitatory neurotransmitter glutamate (glutamate receptors) such as the NMDA receptor and AMPA receptor are overactivated. Excitotoxins like NMDA and kainic acid which bind to these receptors, as well as pathologically high levels of glutamate, can cause excitotoxicity by allowing high levels of calcium ions (Ca2+) to enter the cell. Ca2+ influx into cells activates a number of enzymes, including phospholipases, endonucleases, and proteases such as calpain. These enzymes go on to damage cell structures such as components of the cytoskeleton, membrane, and DNA.
Dr. Drew said it can happen 10 years after people take a whole bunch of LSD (exactly how much is unknown.) It “seems to burn out the arousal system, the part of the brain that feels good, because it’s over stimulated.”
Adam described it as, “no free rides in nature.”