Marijuana has serious long lasting effects. This study and several other articles based on the same data have shown without any ambiguity that it has long term serious effects. As an epidemiologist I attended a conference in July 2014 where a study was presented on this same cohort. This study followed 1000 new-zealand young middle-class for 20 years from 10 to 30. Accounting for parents family situation, family income, school grades etc. they show unambiguously that use of marijuana is linked to:
- increased occurrences of mental health issues such as depression
- lasting mental deficits
- deteriorated relationships (couple, friends)
- lowered feeling of happiness in life
- lower professional success (typically lower incomes at same age than their parents, the opposite is true for non marijuana users)
- poorer financial situation (lower credit scores in particular)
In addition, higher consumptions are linked with worse outcomes.
It is also well acknowledged that the onset of schizophrenia is very connected to marijuana use. It's difficult to know which way this is working but probably both ways.
A fairly synthetic description of the impact of marijuana can also be found on this University of Washington page.
Finally, I have two personal anecdotes that I will make sure to tell my kids before they come of age.
I had a girl friend who smoked a lot of marijuana for from 17 to 19. I met her when she was 20, she had completely stopped for a year. She told me that was 2 lost years in her life, she did basically nothing and feels like she will never completely recover. She was brilliant in school and was now painfully getting the minimum done to finish her studies.
The cousin of a friend had the worse experience of all: he smoked once, got high and never got down, he will likely be unable to do anything in life due to severe hallucinations. He for sure had specific genetic predispositions but who knows his or her genes?
Given all that, my message to my kids is clear: you cannot afford to take the risk.