Getting drunk or high opens one up to the possibility of doing any number of gravely harmful things.  One risks endangering others in serious ways (DUIs), one risks engaging in immoral and risky conjugal activities, one greatly endangers oneself and forces your friends to take care of you.  Losing the ability to reason is a grave sin in itself and opens one up to commit any number of other sins.

Secondly, most recreational drugs have serious health risks.  While it is usually ignored, we do have a moral obligation to take care of our bodies and our health.  This comes from the natural instinct and duty of self-preservation.  Our lives are not our own—they are gifts from God—and other people have a claim on us as well, such as our families and friends who rely on us.  Thus, we must protect ourselves and try to stay alive.  This includes maintaining general health.  So, doing acts, for the sake of fun, which can gravely harm our health is immoral.  Most recreational drugs are dangerous—indeed, this is the major legal justification for their being illegal!—therefore, using recreational drugs is immoral.