Niacin, also known as vitamin B3, is a vitamin that the body uses for over 50 known chemical processes. One thing it does is help the body break down fats and cholesterol. Some people believe that since it speeds up fat metabolism a little that it may help speed up the process of eliminating THC from the body. People have tried taking large doses to try flushing out their system before a drug test.
There is no evidence at all that this works. Doses of niacin over 2 grams can be risky for the liver. Higher doses of niacin can also cause uncomfortable skin flushing and warmth. Drinking lots of water and cranberry juice may speed things up a little or help dilute your urine, but the only way to be sure of beating a drug test is not to use drugs before taking one or using one of the various pre-packaged products available.