When you take powdered nystatin, it will kill candida beginning in the mouth and all the way through. It is not absorbed into the bloodstream. However, other antifungals are, so if you are taking oil of oregano, etc., that would take care of those concerns.
You aren't likely to have a candida problem without it starting in the gut, so nystatin is very helpful. It has done me a world of good. I have taken other measures, as well, which I have already written about. Feel free to look back through my posts if you are interested.