Quote Originally Posted by Burnt Toast
Niacin is an old myth that has its origins from a very unscientific "study" conducted by L. Ron Hubbard and the Scientology Movement more than four decades ago. Hubbards so-called "study" however, was centered around a certain (off-topic) hallucinogenic drug. He claimed that ingesting niacin and sitting in a sauna would, over time, eliminate the "residues" of the drug and prevent "flashbacks". There is not a shred of scientific evidence to support Hubs ludicrous claim.
Hub was a crackpot ex-science fiction writer who had no medical training whatsoever, charges $25,000 to join his sect, advocates total silence during childbirth, and claimed (along with his chief minions Tom Cruise and Brooke Shields) that niacin can cure anything from post-partum depression, to toxic-shock syndrome. Therefore, anything that comes out of Hubs mouth should be taken with a box of salt.

And if that wasnt enough, somewhere down the line some "brilliant" pothead, obviously acting under a garbled understanding, took Hubs claim and thought "hey man, if this niacin thing can get rid of the residues of this (off-topic) drug, then it must get rid of THC residues" ... What Mr. "Brilliant" Pothead and Hub never realized, is that drug tests look for drug metabolites - not residues.

The myth had been born.

And a myth that still persists today because there are many people that claim to have taken niacin before taking a drug test and passed. The niacin was wrongly credited for their passing when the real hero was the copius amounts of water that theyve consumed with the niacin, which effectively diluted their THCA concentration in their bladder to where their urine sample registered below the cutoff thresholds of the drug screen. These people wouldve passed anyway, just on the water-loading alone and without the niacin to go with it.

With all that being stated, niacin does nothing to help pass a U/A.
And besides, taking considerably more niacin than the 15 mg RDA can have some serious health consequences:

Misusing Vitamin To Foil Drug Test May Be Toxic; Plus, It Doesn't Work
and
Morons Overdose on Niacin While Trying to Beat a Drug Test | Wired Science | Wired.com