I was reminded of the adage, "Correlation does not imply causation," when I read this abstract.
This
paper investigates whether cannabis use leads to worse mental health.
To do so, we account for common unobserved factors affecting mental
health and cannabis consumption by modeling mental health jointly with
the dynamics of cannabis use. Our main finding is that using cannabis
increases the likelihood of mental health problems, with current use
having a larger effect than past use. The estimates suggest a dose
response relationship between the frequency of recent cannabis use and
the probability of currently experiencing a mental health problem. [emphasis added]
Many of the serious potheads I have known have struck me as people who would have had mental health problems with or without cannabis use.