I've seen firsthand what drugs can do.
He had the most beautiful of minds. Arrogant, cocky and intensely brilliant. He need hardly lift a finger and he would have had the world in his hands. We were best friends. You did not get in an argument with him or even stray toward an intellectual topic if your opinion differed from his. He could demolish people with the facts and figures he pulled from the recesses of his mind. His mind was incredible and his potential was limitless. For a while he said that the marijuana enhanced his thought process and capability for learning. It seemed like it too, for a short time. I don't know when he started using heavier drugs, but I know now that marijuana, though shockingly horrible to me, was child's play for him, a mere stepping stone for what he eventually got into. The changes came so gradually that I hardly noticed them at first, it took nearly two years to realize it. They started corroding his brain, from the bottom up. His grammar and spelling, once so impeccable, starting stumbling and falling short. Their-they're-there, you're-your, two-to-too, the simplest of things once so routine and expected from him deteriorated. Punctuation turned around completely, disappearing altogether at some point. His arguments, usually so seamless, became less logical and nearly comical at their simplistic incorrectness. He still retains information and can formulate ideas gorgeously but not nearly as stunningly as once before. After barely graduating high school he forsook the many college scholarships and opportunities he had to work at a ski resort. He severed ties with most of his family. He began taking courses from a community college but dropped it soon after. I couldn't tell you what he does with his spare time anymore, my best guess being getting high. It's a hobby, I suppose. I've seen him high. I was there once when he did. He left the room and when he came back it was such a difference. His eyes, usually so bright and perceptive, had dulled and dilated. It seemed like part of him wasn't there but the other part was struggling to put on an act like he very much was there. Phony act, we saw right through it. I never would have noticed it myself, a friend had to point it all out to me. I'm much too oblivious and naive. Once my eyes were opened to it though, I took notice of it all. The way he became so relaxed, very different from his usual, bouncy self, how he stood in a corner and idly chatted instead of raucously socializing. He changed. He always says that drugs have no side effects, they aren't harmful. Every time I see him or think of him I see a beautiful mind wasted. I see just what they did to him. They're eating him from the inside out. It won't be long before he can hardly manage a coherent sentence, let alone debate entire roomfuls of people. Soon the bubbly, hyper boy I knew will be annihilated completely and the mellow, benign, drugged-up boy will have taken over. The drugs took my best friend.
Although, it is the choice he made to use them that took my best friend from me, not the drugs themselves. 
The consequence of that action will forever haunt me.