This site is designed for parents and youth as a 3 step plan:

#1 Learn... Be prepared by knowing the facts about drugs. Watch for signs of use and recognize drug paraphernalia.

#2 - Evaluate. Perhaps you've found drugs in your teens room, or witnessed a friend using drugs, or you've noticed a major change in behavior  that might create a suspicion of drug use... DON'T IGNORE IT. This is a critical time.

#3 - Take Action - Monitor your child, watch your friend. Have a few conversations. If it's your child ask them, Who? What? Where? and When? regarding activities. If its a friend,  let them know you don't do drugs or alcohol, and that you'd like them to stop before they get hurt.

When you get a good idea of the situation, decide the next step.