Niacin is a vitamin that helps speeds up your metabolism. It is believed that taking large doses will speed up the elimination of THC from the body and produce a negative result for marijuana on a drug test. Personally, I think a combination of your metabolism, body fat, activities and smoking history will help you realize how long it will take to clean out your system naturally. Niacin can help, though many companies test for it in drug test. You usually should stop taking the supplement 1-3 days before the test.