IMO, the two most powerful psychoactive substances known to man are produced naturally in the body. I’m sure others have a different opinion regarding what are the most powerful substances. I choose these because they are so very potent and can have both good and bad effects.
Dimethyltryptamine - Made by the pineal gland.
Testosterone - Made by the testis and the adrenal gland.
These are not deadly substances, but their widespread and powerful effects are simply awesome, and sometimes terrifying.
DMT is supposed to be quite a wacky time.