If he seems defensive or assures you that he doesn't know anyone who uses drugs, ask him why he thinks people use them. Discuss whether the risks are worth what people may get out of using them and whether he thinks it would be worth it to take the risks. Even without addiction, experimentation is too great a gamble. One bad experience, such as being high and misjudging how long it takes to cross a busy street, can change, or end, a life forever. If something interrupts your conversation, pick it up the next chance you get.
Newspapers are full of the consequences of alcohol and drug abuse.
Take your examples right off the front page. Ask your child if she heard about the mother who used drugs and was arrested. Watch TV with your children, and ask them what they think. Whenever you see an anti-drug commercial on TV, use it as an opening to talk with your children about drugs. Ask them what they think about the commercial.