Well just my opinion, it very well could be seratonin deficiency or could be the fact that barin cells are overstimulated and the cell dies which leaves less receptors which means less seratonin. Drugs overstimulate brain cell receptors causing them to literally burn out. However, it's not the only thing that does. Watch this video, it is a must watch video about excitotoxins and how they are hidden in almost every food.