It can cause anxiety disorders. I have also heard that some people can become psychotic with hallucinations, paranoia, and delusions. If you are prone to psychosis, it can bring it on, especially if you smoke it a lot.
Just search for "marijuana psychosis" and you will see some research on it from some very reputable journals and news websites.
If you are experiencing psychosis, you need treatment for that and for the marijuana use, which will only make it worse. Quitting won't be easy, but it's easier than having a psychotic break-down.
