How to p a MARIJUANA test, but NOT a narcotics test?!?

I need to clean the Marijuana out of my system, but not the Narcotics that I take. It will seem suspicious to my doctor (who has a no weed policy) who prescribes Narc's for my pain. All the cleanses clean everything out of your system. I've heard Certo (fruit pectin used for canning) mixed with Cranberry juice works... HELP!