It may cause your face to flush 10-20 min. after ingesting, it slightly blocks the breakdown of body fats, may reduce oily skin, and it may prevent Alzheimer's-like symptoms.
In doses over 2-3 grams daily it can cause liver damage, elevated blood sugar (dangerous if you are diabetic), blindness, and even death.
But what it doesn't do is keep you safe from drug tests.
IF you are a very light user then your body should clear itself in 2-4 days.
If you are a habitual, heavy user it takes 4-5 weeks.
The only thing niacin does is partially block the breakdown of body fat, and since THC binds to body fat, if your body is running a bit low on blood sugar, niacin may help resist the breakdown of that fat, which would lead to additional THC being dumped into the blood.
So additionally, do NOT fast nor exercise during the 24-hrs preceding the test.
And maybe start thinking about quitting. If and when you decide to quit, if you find it difficult (and you probably will) call up and go to MA meetings.
I'm just saying...