Niacin works, but not as an entire course-of-action, but as a supplementary to the work you are already doing. You should also try to stay clean for an extended-period prior to drug testing. I was actually going to write a detoxification guide, but I never quite got around to it, or really found enough time. But here is a good website with a lot of information about Niacin and its role in bodily detoxification.

The role of niacin in detoxification