A mental disorder is any disorder that affects the function of the brain making the patient incapable of normal life behavorior. Depression, anxiety, dementia and Schizophrenia are just a few. A mental disorder interrupts the normal function of the brain cause a multitude of issues and conditions. In order for a condition to be labeled mental disorder the condition has to cause dysfunction in the patients life.
        
             
        
        
        
I just had a big test on marijuana, steroids and inhalants 
One of the reasons a teen would choose not to do marijuana is because it can cause you to have short term memory lose. It can also cause you to loose you’re coordination. It also can cause some one to have lung problems. 
Hope this helps you have a great day!!
Good luck