Not sure if this was mentioned before...
Marajuana (sp?), cocaine, steriods, and other drugs began as medicine. Later on, people found out about the side effects, and some bastardic idiots began to abuse those side effects (or effects) for their own pleasure. Your friendly neighborhood FDA and US Government found out, and banned over usage... blah blah blah.
To make a long story short, people use drugs to feel good. Others use it for medical purposes, but overdose themselves (poor fools). Even more people crank out the stuff (illegally, most of the time), and get rich (and/or arrested).
Drugs=pleasure, medicine, money, stupidity, jail time, and eventually death. This leads to the answer to your question, but it leaves ME a question.
Why do so many humans abuse drugs, even when they know of the things that can happen?
And that leads me to the question: Why? (A.K.A. the unanswerable question that always leads to the question: Why?, A.K.A. UQTALTTQ:W?)