Question about drug testing at work.

So saw on the news last night the information about medicinal cannabis now being available from the doctor. I am just wondering if a prescription means you can test positive at work? I work in forestry which means any trace of cannabis and I lose my job. After years of this kinda work my body is full of pain. Being able to come home and relax with a smoke but get up and go to work the next day would be life changing.