I was really thinking of the long-term pyschological damage that smoking too much weed may do. I know that isn't the case for every single weed user (i.e AzLaker), but, like smoking 20 fags a day for 40 years & risking lung cancer, it's a gamble, it's russian roulette.
There's also the obvious fact that smoking joints, esp. those mixed with baccy, can lead to cancer, heart disease, emphasyma, etc, but I know there are ways to enjoy cannabis nowadays without those risks (vapouriser), which I think is great.
I guess that being someone with a relatively serious mental illness, I get somewhat worried when I read that long-term users often get serious mental health problems later in life. That is probably why I only like to have a smoke occasionally; maybe I'm wrong for automatically believing these government-funded health reports, & the book recommended earlier sounds like something I ought to check out ASAP.
Peace.