Most of it goes down to the psychological effects it has on you. It effects your short term memory big time and influences your empathy / understanding of the current situation. Depending on it, there is a chance that it can be harmful for your body too. Since I stopped 2 years ago I noticed that I started twitching. Not regular but once in a while. I am pretty sure something is -blam!-ed up with my nerve system since it just happens out of nothing. Plus your lungs -blam!- up since it's nothing different then smoking. (clogging your lungs with shit that's bit supposed to be there)