Re: alcohol or marijuana, effects different people in different ways, and if one is predisposed to the effects of it moreso, will react or act very differently to say, the next person.
The only trouble is, the younger a person is when using pot, or alcohol, the more it is likely to effect their maturing process.
Marijuana does have beneficial medicinal properties, if used correctly and for the right reasons, but as with anything, nothing exceeds like excess, and then....one becomes unbalanced in many ways.
Those that I know that smoke pot, have done so for years and can't shake the habit, same as it those that have been drinking for years.....name your poison, and there is your addiction, mine happens to be chocolate...ah yes, death by chocolate