Your question was asked of Rabbi Moshe Feinstein, zatzal, regarding Marijuana. The following is a summary of his answer:
The Torah teaches about a 'Ben Sorer Umoreh' - a rebellious son. A Ben Sorer Umoreh is a youth who steals meat and wine from his father and gobbles it down. The Torah calls for the death penalty in such a case. Since he is addicted to physical pleasures, he will ultimately do anything to support his habit, even rob and kill. The same is true of drugs, and more so. People who use drugs usually acquire a strong hunger to maintain their habit, and can develop physical and/or psychological dependence.
In general, using drugs is unhealthy. But even if no harm is done to the body, drugs lead to a lack of concentration needed to pray, fulfill mitzvot and learn Torah properly.
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, the Torah says "Kedoshim Tihiyu" - "You shall be holy" - meaning that you should not seek out and indulge in excess pleasures. Based on all the above reasons Rabbi Feinstein prohibits Marijuana [and urges educators to do all they can to dissuade people from using it.]
Now if you ask, "But what about alcohol? Don't all the above reasons apply equally well to it?" The answer is... "Yes!" In the words of the Rambam, "Someone who gets drunk is a sinner." Alcohol, when used for the purpose of getting intoxicated, would be in the same category as drugs.
Another point: Using drugs, even for 'recreation', brings you in contact with 'professionals': Users and - unless you 'grow your own' - dealers. So, as they say: "If the drugs don't getcha, the company will..."
