The evidence for a link between Cannabis and increased risk of psychosis is increasing [1]. A patient with psychosis rarely recovers and is extremely costly to society over his lifetime (as well as a personal tragedy, of course). I'm in favour of legalisation but it is something to take into account.
[1] http://www.psychiatrictimes.com/schizophrenia/content/articl...
From that article:
"""Conclusions Despite all of the uncertainties surrounding the cannabis-psychosis link... The evidence suggests that cannabis is associated with an increased risk of psychosis when it is used frequently. Whether cannabis can trigger a primary psychotic disorder that would not have otherwise occurred is unclear.
However, in most individuals who use cannabis, psychosis does not develop, which suggests that the increased risk must be related to other vulnerability factors (genetics, frequency, or age of onset of cannabis misuse)."""
Yes, but only to those who are already disposed.
That having been said, I think that it is different than what the GP was referring to which is a chemical addictive property (like nicotine) rather than a long/short term side effect (smoking side effects, whether it be tar in your lungs or increased risk of schizophrenia)
[1] Sorry for the language, but the biggest joke of this entire debate is that Cannabis is a schedule 1 drug.
There is a lot wrong with this bit here. First, I'm not sure where you are finding a figure that cannabis use will double post-legalization. The actual count of users of cannbis isn't fully accurate due to the illegal nature of using it and places that have purused legalization or decriminalization efforts have actually seen a falling trend in cannabis consumption.
Second, the effects of regular pot use ARE NOT like alcoholism or smoking. Cannabis does not have a physical addiction characteristic like alcohol or cigarettes, the method of operation on the body is completely different from alcohol and cigarettes, and research on the health effects of cannabis have been extremely limited due to the schedule I status of cannabis in the US. Granted there may be health issues associated with cannabis usage, but the problem is not well understood and will not be well understood so long as cannabis is schedule I and research on it restricted at the federal level.
Third, legalizing cannabis does have a real dollar cost as it relates to health care, but this isn't a sign to continue to keep cannabis illegal, but rather to fix the fundamental structural problems of health care in the US. Also, it is not clear that there is a causal relationship between some types of cancer and regular cannabis use, and amounts of cannabis consumed by regular users is highly variable. Further, combustion is not the only way to consume cannabis, so the association here with cannabis being a smoking type drug is quite wrong, and in fact many folks with smoke sensitivity do not consume cannabis this way at all.
I agree that like any legal change, there are costs associated with legalizing cannabis, but those costs are so much cheaper than what we have going on today in the US and in the countries where the war on drugs has resulted in tremendous violence and political instability.