Sooner or later all young people will find themselves in a situation where they are offered drugs or alcohol. It is important for them to be able to distance themselves and say no. The best protection is your child developing courage, self-confidence and personal responsibility.
There is no point in talking to young children about the dangers of drugs. When they are older, though, it is worth talking to them about addiction and the harmfulness of tobacco, alcohol and drugs. Factual information is more helpful than threats or preaching.