Drugs don't kill people. Stupid people kill themselves. There are plenty of ways to enjoy recreational drugs sensibly and responsibly, just like alcohol. (In fact, alcohol is a recreational drug in most contexts.)

Instead of a "Don't do drugs, they'll hurt you!" attitude, try to be more understanding. Convince your friend to set a limit, make sure he's educated about the drugs he uses and drug use in general, make sure that if he ever feels that he's addicted, he talks to someone about it or tries to get help, and most of all, make sure that you make it clear you support his decisions on how to live his life. There's nothing worse than having a friend that you feel doesn't understand you, especially if that person could be a pillar of support if you ever get yourself in a bad way.

It would be sad if you lost a friend, but it would be much sadder if you find out he's dead three years after you broke it off with him, just because you weren't willing to be there for him, even if you don't agree with everything he's doing.

And as for personality change, it happens. Just like rivers, you can't step into the same one twice. But friendships, really good ones, are always supposed to be there.

I suggest you read up on drugs and drug use yourself, and not from just one side. The foremost drug community and knowledge resource on the web is easily erowid.org. If you are educated and understanding, you can help your friend to become educated, and therefore safer.

I also implore you, on a personal note, to please abandon the "Drugs will ruin your life," standpoint. As I've said earlier, there are many responsible ways to enjoy drugs. Moderation, damage control, and education are all very important to responsible drug use, and if properly implemented, can reduce bad happenings drastically. It's just a shame we live in a culture where such preventions cannot take place.

If you've got any questions or comments, feel free to respond, and I'll try to take care of them the best I can.