Everything is poisonous at high doses even oxygen.
"[E]ven oxygen" is funny because it is absurdly reactive and will explode when given the opportunity, and way back when it destroyed pretty much all life on earth except the early cyanobacteria that produced it.
Edit: And yeah, I didn't imply what you're responding to, I described the mechanism for water poisoning.
It's one of the risks with taking MDMA, there are some documented cases where people died from drinking too much water when rolling.