Niacin is an ancient myth. A myth that still persists today because of people who had taken niacin before passing a U/A. The niacin was wrongly credited for their passing when the real hero was the copius amounts of water that theyve drank with niacin which, in effect, diluted the THC metabolite concentration in their bladder enough to where the urine sample registered a negative on the drug screen. These people wouldve passed anyway just on the waterloading alone and without the niacin to go with it.

Its all about the water.

And drinking a bunch of water days or weeks before a U/A is of no help. THC is not water soluble like other types of drugs - its fat soluble. THC is stored in the fat cells and remain there until the fat cells are burned for energy. Therefore you cannot "wash" or "flush" THC from the fat cells like you can wash dirt off of your car. It does not work that way.
When fat cells are burned, the stored THC metabolites are released back into the bloodstream. From there, the kidneys filter the THC metabolites from the bloodstream and are deposited into the bladder for the eventual exit. The rate in which the THC metabolites are filtered from the bloodstream by the kidneys is unaffected by fluid consumption.

Drinking fluids is only helpful on the day of the U/A. The purpose of drinking water is to dilute the THC metabolite concentration that is present in the bladder in the hoping that the urine sample would register below the cutoff thresholds of the drug screen. This dilution phenomena, however, is only temporary - once the fluids ingested had all been urinated out, the bladder is replenished with fresh THCA-laden urine and thus, the donor would resume testing positive for THC.

There is no product that can rid the body of THC. The only proven way to permanently rid the body of THC is time and fat-burning exercise.