I took a course on drugs and society in undergrad simply because I wanted to have an intelligent and coherent stance on this issue. And my answer is still: I don't know.
on the one hand I do agree that people should be allowed to abuse and destroy their own bodies if they so choose, however they want to do it. If you want to d,o it by abusing drugs or alcohol, then as a libertarian I say "Go ahead." Peyote? Weed? Tobacco? Caffeine and alcohol? makes no difference to me as long as you are affecting primarily yourself.
The catch I see is twofold:
1. I do think drug use (especially harder drugs) does entail a societal level harm. Nobody quite knows how much and estimates go back and forth, a lot of screwy numbers flying around out there, but my take from the numbers I've seen is that it does include serious societal level harm. And while I'm ok with you hurting yourself it is when you hurt others that it is correct to make legislation (imho).
2. The line has to be drawn somewhere and no matter where it is drawn nobody will be happy and will pick at it. I think that's just a part of human nature. Sometimes there isn't a good place to draw a line. And if there isn't a good place to draw it, and the line has to be drawn (which most people agree with in my experience), I'm hesitant to move it.
I'm not sure that decriminalizing it is the correct tact, I do agree that much of the "war on drugs" has been big on the political capital and short on the useful effects while wasting an incredible amount of money. I don't want to waste money in a futile effort. On the other hand making it legal won't solve everything associated with it.
All that to say: I don't have a clue how to deal with weed
What I'd like is a coherent policy on drugs. This is legal because xyz and this one is illegal because abc. But I haven't heard any such proposition.