Q. Now that the use of recreational marijuana became legal here in California (or is about to, as in other states), is there a Halacha issue for not using it, even when not in a smoke? Why should it be different than liquor?

A. Igrois Moishe, (Y. D. 3: 35) writes that the use of recreational marijuana or hashish is prohibited because it implies transgression of a number of main principles of the Torah, such as endangering one’s body and mental health. He adds that it is also detrimental to the learning and clear comprehension of Torah and the correct kavana and intention in the observance of mitzvos.
Horav Moishe zt’l compares the use of drugs, to what the Torah teaches about a “ben sorer umoreh” or the proverbial rebellious son. This son is a youth who steals meat and wine from his father and hastily gobbles it down. He became addicted to physical pleasures, and he will ultimately do anything to support his habit, even rob and kill. The Torah calls for the death penalty in such a case. The same is true of drugs, and then some. People who use drugs usually acquire a very strong desire to maintain their habit, and can develop physical and psychological dependence.
The Rov zt’l adds that parents are usually distressed by a child’s drug usage, so using drugs could lead to violating the commandment to “Honor your father and mother.”
And finally, he asserts, that the Torah teaches “Kedoshim Tihiyu” – “You shall be holy” – meaning that you should not seek out and indulge in excess pleasures. Founded on all the above reasons Horav Moishe forbids the use of recreational drugs calling it a severe prohibition and urges everyone involved to do all they can to dissuade others from using them. (See Assia Journal – December 1983, in regards to smoking in general, including recreational drugs and the opinion of Horav S.Z. Auerbach zt”l)
The same would apply to the excess use of alcohol.
Horav Shlomo Miller’s Shlit’a opinion is similar. The Rov added that the use of recreational drugs may in addition to all the above proscriptions, place one in the company of very undesirable individuals and their most damaging friendship and influence. It also often comprises a catalyst for family difficulties and problems.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a