fact #1) doctors advise people with high cholestrol to use small amounts of niacin daily in order to increase their metabolism, and therefore increase their body's ability to metabolize fat.

fact #2) THC is stored in fat cells

Can Niacin help you pass a piss test? Yes. Can you pass after smoking the day before if you use niacin? Highly unlikely.

By the way, there is such a thing as "flush-free" niacin.