Methamphetamine is not as toxic as some may think. Yes, it does run down your body, but thats mostly due to the fact that most dont eat or drink and neglect their nutrition while high. ITs a serotonin, dopamine, and addrenaline releaser, and also acts as a SSRI (selective serotonin reuptake inhibiter).
The other problem is crack heads make it. As a Biochemical Engineer, production is harmless with no risk of blowing anything up; and the end result is very pure with no harmful leftovers like lithium hydroxide, hydrochloric acid, acetone, ether, etc. Those byproducts that "cooks" dont know are there and dont care to extract are what cause most of the damage. If u mix blood with these things, it destroys the cells and creates black tar-like clumps that can reak havoc in your circulatory system. If its pure, nothing happens and when its over it just wears off; no painful crash. Obviously you cant do it every day or several times a week, but not everyone is weak and cant control themselves like I can; or create the best product ever.
, which controls small intestine function i.e. cramping in smooth motion to push waste through which is why some get cramps after doing it. It's also responsible for mood regulation as well as starting your sleet cycle, which explains why people dont sleep and get moody when it wears off.