R. Barry Freundel, Contemporary Orthodox Judaism's Response to Modernity, p. 251:
At times, people raise the question of why alcohol may be acceptable in some situations but marijuana is not. This question has become less frequent as the negative mental and physical effects of marijuana and its bridging effect, which leads many users to other drugs, have become more obvious. Nonetheless some comments should be made on this subject.
First, marijuana is illegal, whereas alcohol is not. Even if one disagrees with this public policy, respect for the law is an important Jewish value. By the same token, being a good citizen may be a halakhic requirements, and avoiding profanation of God's name by being identified as a Jew who violates the law is certainly an obligation.
Click here to read moreOne other critical difference between marijuana and alcohol must be highlighted. Drugs comes with a drug culture that is not at all the same as the culture surrounding alcohol. The drug culture sees the altered mental state that one achieves under the influence of narcotics as a desirable outcome. New insights, better perspectives, clearer and more heightened awareness of reality are claimed to be the product of drug use, at least in some circles...
When it comes to harder drugs than marijuana, all that has been said so far applies. In addition, the greater health risk, and the danger of addiction (which can occur, at least as a psychological dependency, even with marijuana), simply puts these things completely outside the pale as far as Judaism is concerned.