It’s assur. Rav Moshe has a good teshuvah about it–can someone post it in English, please?
Some people may appear to use it without negative consequences, but in fact, for many people it tends to have many negative effects — anxiety, paranoia, panic attacks, laziness, lack of motivation, psychological dependence, lack of normal physical and emotional maturation, focusing on maximizing physical pleasures and desires, etc. A large proportion of users have serious anxiety problems from it. Several studies show that young people who use it have twice the rate of schitzophrenia and psychosis–which by the way, are incurable.
As Rebbe Nachman said about tobacco–what, do we not have enough cravings in this world that you need to add another one?
Just think of how many tzaddikim have lived b’simcha throughout the ages without ever trying marijuana (or wanting to)! Davening, fasting, a little l’chaim — we Yidden already have plenty of time-tested ways to alter our consciousness. It’s not necessary and it poses dangers — so why try it?
At the same time, yes, it should be legalized, since the Torah frowns on prison as a punishment anyway, and there are better ways of reducing drug use and dealing with addiction than locking up addicts in prison. But that doesn’t mean we should use it.