I currently work in a job where I am required to have a security clearance and I have had one for five years. I have chronic severe pain and have been advised that medical marijuana is the best option for treatment of my pain. I live in a state where medical marijuana is legal. I know if I decide to try this that I will need to report it. What happens next and will I be able to keep my security clearance?
Marijuana remains illegal at the federal level, regardless of what any particular state may declare. You would absolutely be risking your clearance by using the drug. While I'm not an adjudicator, current use of an illegal substance while possessing a clearance will likely result in an automatic denial and possibly the cancellation of your current clearance if you report as required.
That said, I'm sure a lot of people are sympathetic of your situation, and hopefully federal laws/adjudicative guidelines will be updated in the future to reflect the medical acceptance of marijuana. You must decide if the drug offers enough benefits to your health condition that would be worth surrendering a clearance over. It's by no means an easy choice, because as of right now they are incompatible. You must choose the best option for you.