In certain circumstances or until a certain age it can be good and have the desired effect to forbid things. In the case of forbidding drugs the effect of the prohibition depends on how important it is to your child. The risk that your child will start to use secretly is large, especially when it grows older. It is impossible to lock up your child in order to protect it from the outside world. You can try, but the question is whether your child will be any happier in a protected life. A child wants to discover; this is part of growing up. It looks around and hears and sees all kind of things that are new and exciting. You are not always present and cannot protect your child against friends that tell him how 'cool' it is to smoke dope and how 'relaxed' it makes you. Your child will think: there must be nice sides about smoking dope. And that makes it curious, despite the fact that you told it repeatedly that drugs are bad and that it is not allowed to use drugs. The risk that your child wants to try it eventually is large, as well as the risk that it hides it from you. After all, you don't understand that using drugs can also be nice.
It is better to make it a subject of discussion and also talk about the reasons why people use drugs. Show your child that you are up-to-date with regard to alcohol or drugs. In relation to this conversation you can make deals with your child and guide it in making choices. Eventually, it is your child who will have to decide about whether to use drugs or not.
You are not always there to protect your child against that. It has to do that itself, by learning how to carry responsibilities.
This subject is also a matter of discussion at school.