Well despite a lot of people saying otherwise I have personal experience that says long term use can cause problems.
One of my cousins, who is about my age, has been a regular smoker since he was in his teens. So that is about 45 years. Today his brain is pretty much fried and he has lost touch with reality. Very little of what he says is coherent and he has developed an extreme paranoia to the point where even his own family members cause him to become extremely nervous and hide out. While he doesn’t have lung cancer that I know of he does have emphysema, which is another smoking related problem. He is addicted to the marijuana just like a smoker is addicted to the cigarettes. The main thing though is that it seems to have affected his brain and just talking with him you can tell he isn’t all there. I know of many recreational users who don’t’ seem to have problems but my cousin treated it like someone else would treat cigarettes and was a fairly heavy user. I am pretty sure that if he hadn’t used it all those years like he did he wouldn’t be in the condition he is in. He does not use other drugs (cocaine, acid, meth, etc) that I know of and his family hasn’t detected the use of anything else either. So I believe that long term, regular use does cause issues with a persons health and mental state.