Also to be considered is that any substance can be harmful if abused in excess; even "harmless" substances or beneficial substances. Even drinking too much water can lead to "water poisoning" [1]. It's even led to a death in a radio contest [2]. There's also the fact that not every person responds to every substance in exactly the same way. Some may experience adverse effects after minor exposure, where others may abuse a thing in excess and never display any outward signs of a negative effect.

[1] https://en.wikipedia.org/wiki/Water_intoxication

[2] http://www.thesmokinggun.com/documents/crime/hold-your-wee-w...