I have no idea! Several people in my street smoke it, and I HATE the smell. Every time I go into my little market town I can smell it. There is ALWAYS someone smoking it. It's exasperating!
Where are they all getting it from? 
They make out it's 'harmless' but it is a gateway drug. I won't get into an argument with anyone about it when someone tries to defend it. If someone is the kind of person who will defend smoking weed, I want nothing to do with them.
2 people (a couple) who live in my road smoke it, like 6-8 times a day, not just one or two at the weekend or something, they smoke LOADS. They are in their late 20s, and when they cough, it sounds like a 90 year old man who has smoked for 75 years. Dreadful! Real nasty death rattle kind of sound! I have known them to cough and cough and cough for half an hour non-stop, often bringing up phlegm. 😖
Their lungs must look like 2 slices of stringy, badly-burnt bacon. Only God knows what their health is going to be like further down the line, and what state their respiratory system will be in by the time they are about 45. (If they live that long.)
Even as little as a decade ago, I don't recall there being anyway near as many weed smokers. As I said, where are all these people getting it from? They can't all be growing their own!