STOP the lies.
If your teen does smoke Cannabis - GET OVER IT! I could be worse. THEY could be drinking and driving. I read of a young lady who killed herself after her mother had been made aware that she was smoking cannabis. There are much more serious problems to worry about - how about just the rise of teen suicide? But to blame them - tell them they will never amount to anything? WTF? Over a plant. That is the crime. Not the use of a plant that perhaps brings them just a bit of peace and relaxation and fun! YES fun is important. Teens and young adults have even more pressure of on them than ever before. So - This one is just out the window.
I know many many successful people - who all smoke Cannabis.
If you go nowhere with your life you have one thing to blame...