i dont' doubt that either but any psychotropic drug whether alcohol or pot or drugs can hot wire or short circuit brain function.......not to say the brain won't compensate but the chemical pathways can be muted or renderd immature by excessive chemical introduction.....
We basically have two choices. One, we can become a prohibitionist police state, accepting a high level of organized crime. Two, we can allow people to self-intoxicate freely with the understanding that no matter what they do they are still responsible for their own behavior. The evils of the former option exceed those of the latter by several orders of magnitude.
The other thing I'll note here is that anti-drug arguments are pretty much identical to anti-gun arguments (people can hurt/kill themselves therefore the government must stop people from having them). If you agree with the reasoning for the former, there's not much ground to stand on opposing the latter.