Ahhh, the old niacin myth. It has ties with the sauna myth. The myth had its origins from the study L. Ron Hubbard (the father of Scientology) conducted more than 40 years ago. Hubbards study, however, was centered around LSD use and the attempted prevention of LSD flashbacks that ran rampant at the time. Hubbard claimed that ingesting niacin and sitting in a sauna would, in time, remove LSD residues and prevent flashbacks. There has not been a shred of scientific evidence that can substantiate Hubbards claim.

Then some "brilliant" pothead, obviously acting under a garbled understanding, took the claim ..."Hey, if this gets rid of LSD residues, it must also get rid of THC residues"...What needs to be understood is that drug tests look for drug metabolites - not residues.
The myth had been born.

And a myth that still persists today because of people that have taken niacin before a drug test and passed, only to not realize that it was the large amounts of water that was consumed with the niacin that brought the urine sample under the tests cutoff limit, thus a pass. The niacin was wrongly given credit when the real hero was the water. Its all about the water.

Hope this helps to debunk the niacin myth.