Many teens believe they are invincible, and death or addiction will not happen to them. At the very least, drugs will destroy their future. Decide what you will do if your teen starts using drugs. Hopefully you will never have to implement your plan, but having one to fall back on will save you much time and energy if you encounter this crisis. I highly recommend a chemical dependency treatment program.
Many facilities offer both intense inpatient programs, as well as a more relaxed outpatient regimen.
Follow through with your plan of action. If your plan of action consists of admitting your teen into treatment, clearly tell them that if they begin using drugs you will place them into a chemical dependency program. When voicing this, don't threaten, just make it clear that because you love them you will take whatever extreme measures are necessary to save their life and future. Know their friends and their parents. Often teens experiment or take drugs with friends rather than by themselves.