000646:
Good question about alcohol.
Alcohol, if used moderately, can be completely harmless. In small amounts, it is not toxic and has no ill effects. I will note that far more people have issues with alcohol than we care to believe. The incidence of alcohol problems in our community is much greater than many believe. We still soothe ourselves with the saying, “Shikker is a goy.” Unfortunately, it is easy to abuse alcohol, and its legal access does make sure that abuse is still easy and largely inexpensive. But the limited use, such as has been prescribed by Torah is not problematic (except for those with increased sensitivities).
Nothing of the sort can be said of marijuana. It is always toxic, and causes damage with each and every puff. The greatest noted dangers of marijuana are associated with long term use, which is not paralleled with alcohol, for which limited long term use has no effect on health.
I am repulsed by the notion that the existence of anything in creation suggests that it is permissible. Before excusing smoking weed, perhaps you should try smoking poison ivy which also seems to lack any other purpose for existing.