A good time for this discussion is in your child's early teens.
Make the discussion conversational and not a lecture.
Sincerely inquire about your child's point of view on marijuana use. If your child honestly feels negative towards the subject, end the discussion by informing your child that he/she can always come to you about anything.
Only let your child know about your own experiences with smoking pot if he/she seems intrigued about the subject or admits to smoking him/herself. Ensure that your child understands the negative effects of marijuana, including lack of motivation and heightened paranoia.
Answer your child's questions as sincerely as possible. Let him/her know that smoking marijuana did not bring any positive changes to your life.