If you read some papers on the subject it should be plenty apparent that it has adverse effects on the development of young adults, as well as long term use by anyone, particularly of recent high-potency strains.
It's not as bad as other drugs (heroine), and it's worse than others (coffee), but it's not harmless. I'm far from being a prohibitionist, and live somewhere that has (I think) sensible policies (The Netherlands), but to simply put that it's "fairly harmless" as something most physicians agree with is not true. I'd say it's similar to alcohol in terms of its moderate use being possible in a working society - albeit with some negative outcomes for people that overdo it, or do it too early in life.
Edit: there's lots of discussion below about if the studies that exist are trustworthy or not, but since anyone can google for studies, I'll leave a different recommendation to check out the r/Leaves subreddit, and read some first hand accounts of long term and heavy users. It's at least a different type of source and you can make up your own mind about what real users say about it, in case you never encountered it before.
Worth noting our current overdose crisis and general lack of health care in many parts of the country, now the under-prescription of controlled medications- which all helps shift a lot of these dynamics in a direction that might not be seen in other parts of the world.
So I just wanted to add that for a subset of the population, the risks are several orders of magnitude more serious than "lost a few IQ points", as many people are not able to resume normal life (nor indeed, a normal experience of reality) after a psychotic experience.
That being said, I do support legalization, since the alternatives are worse. I just also support people being well informed, and aware that while they're probably not in that 2%, there's only one way to find out, and you really, really don't want to find out.
"
There is substantial evidence of a statistical association between cannabis use and:
The development of schizophrenia or other psychoses, with the highest risk among the most frequent users (12-1) There is moderate evidence of a statistical association between cannabis use and:
Better cognitive performance among individuals with psychotic disorders and a history of cannabis use (12-2a) Increased symptoms of mania and hypomania in individuals diagnosed with bipolar disorders (regular cannabis use) (12-4) A small increased risk for the development of depressive disorders (12-5) Increased incidence of suicidal ideation and suicide attempts with a higher incidence among heavier users (12-7a) Increased incidence of suicide completion (12-7b) Increased incidence of social anxiety disorder (regular cannabis use) (12-8b)"
National Academies of Sciences, Engineering, and Medicine. 2017. The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research. Washington, DC: The National Academies Press. https://doi.org/10.17226/24625.
I would warrant that these summaries should be a concern for anyone using cannabis and that blanket statements regarding the overall tone and summation of the report negating health effects of cannabis is somewhat misguided.
Clinicians aren’t the ones to go to for harms anyways, they’re largely not doing the research at any level.
It has also been confirmed that heavy use of marijuana has negative effects on cognitive performance and short-term memory even in adults, although these symptoms go away after you stop using.
One of the more challenging things with cannabis is it can trigger people who are more predisposed to issues. Some of these things can stick around for a while, after an initial incident. Compared to something, like alcohol, cannabis based issues don’t only affect heavy or long term users. You might just be the unlucky person that cannabis doesn’t jive with.
That being said, I think she largely thinks legal cannabis is good. She’s seen recovered alcoholics who’ve turned to cannabis as their outlet without killing their liver and destroying their body.
However, acting like there are no risks to cannabis is not helping anyone.
I have seen people go into psychosis from weed. & no it wasn't laced. I have seen my gf's dad go from a non-smoker to rolling a blunt every hour. I have had friends drop out of college due to weed.