One can dispute the findings by the study group, but it is a drug. Last year one co-worker confided in me that he was once a heavy user but quit when he started getting paranoid. I won't lift a finger to stop some adult from using the stuff, just don't tell me that prolonged use of any drug has no negative side effects.
I can look back on those youthful moments and realize how much of my future I was really just pissing away. I was always in a fog, no short term memory and no motivation.
Frankly if someone wants to be a drunk or a druggie then that should be their choice but you are correct in saying that there is cost to the behavior.