Imagine that a bag of weed falls out of the jacket of your daughter or you hear from an acquaintance that he saw your son come out of a place where they sell hashish.
Maybe your first reaction is to panic. That is understandable, but as you know panic is a bad advisor. When your child is experimenting with whatever substance, it is important to talk about that. It is the only way to find out how important the substance is for your child, so that you can judge whether your child is running risks or not. If you forbid right away, your child will feel that it has to hide something, which increases the risk that he will experiment out of your reach. An open conversation will lead to a better mutual comprehension and strengthen your contact so that you can let go of your child with a better feeling. Make sure in your conversation that you know enough and are up-to-date so that you are not stuck for an answer. It also prevents you from worrying for nothing. Don't try to force the conversation; a good conversation can only happen if both parties are open to it. Give your opinion but don't scare your child. Forbidding only is counterproductive.
Keep in mind that youngsters are curious and they should be in order to develop. Experimenting with stimulants is almost always part of this. A first beer at a young age doesn't make an alcoholic.
Try to makes rules with your child and indicate your limits. For example only in the weekends, not in combination with homework, schoolwork or traffic. Youngsters need parents that set limits, also in the case of stimulants. The rules give your child something to hold on to and show that parents are interested in them and have attention. Finally it is important to avoid boredom. The use of stimulants can also be caused by boredom. If necessary help your child to order his time off, stimulate hobbies, sports and contacts with other youngsters of the same age. You can also do things with your child every now and then.