Information adapted from The Naked Roommate.
Besides being illegal, recreational drug use is dangerous—you run the risk of becoming addicted or even dying after just one use. And sure, you can die just crossing the street, but when you're high, everything you do becomes much more dangerous.
Drugs take up too much time—there's finding them, earning the money to buy them, doing them, talking about them, recovering from doing them (and if you get busted, you'll also spend time with police, parents, lawyers, and possibly jail).
If you stay away from people who do drugs, drugs will not be a problem.
If your closest friends smoke pot, chances are you'll cave and smoke, too.
If you or someone you know has a problem with drugs, seek help either from your school or from a program in the surrounding community.
The Naked Roommate has more information about drugs and their effects, including advice for those who make the choice to do drugs on campus.