Niacin is a myth and does nothing to pass a U/A. Buy a home THC test and test yourself using the first mornings void to see where you stand. A home test would tell a more accurate story than a bunch of guesswork. If your home test is positive, you should consider subbing (with a clean sample or synthetic) or dilution.

Check out N2's Substitution and Dilution threads in the stickys.