Can marajuana use be linked to mental illness? Or does it help stop mental illnesses

and if so would you have to see a phychiatrist and be treated everytime you decide to smoke to be cured of a mental illness? If so what happens if you never see a doctor? If not what's so bad about the use of marajuana if it doesn't pose a threat to your health? People say they use it because it calms them down, and lessons stress. Is that true?