I broke my shoulder afew years ago and needed two operations, must say I got really sick of taking panadol and other pain relief drugs. So I smoked quite abit of weed during that time. Its good for pain relief.
In San Francisco, until recently, marijuana was available for medical purposes.
There is a mind-set regarding smoking cannabas - some people see it has evil and like you say crap, some enjoy it like people do alcohol. I hardly drink, my dh does not like alcohol. Most people in this life take some from of social drug, legal or otherwise.
I am writing all this so you can perhaps understand more about maijuana and its uses.
I am sure you are well aware that teenagers are just like babies, they can change and do new things overnight. Whilst boundaries within your household are good for respect of all occupants - for me if it was my child, I would always leave my communication door open and not judge even if I wanted to. That way lies and deception are hopefully less able to multiply.
I have always been honest with my parents that I smoke. From being very anti they have learnt flexibility and alot more understanding about drugs. Not to say they 100% accept, but they have learnt it not a one way street to the gutter which they used to think.