FBR is correct in stating that the sauna/niacin myth originates from the "study" Hubbard conducted over 40 years ago. Hubbards "study", however, was centered around the use of a certain (off-topic) hallucinogenic drug and the attempted prevention of flashbacks that ran rampant at the time. Hubbard claimed that ingesting niacin and sitting in a sauna would, in time, remove the residues of the drug and prevent flashbacks. As FBR pointed out, there hasnt been a shred of scientific evidence that can substantiate Hubbards claim.

Then somewhere down the line some "brilliant" pothead, obviously acting under a garbled understanding, took the claim ..."Hey, if this gets rid of (off-topic) hallucinogenic 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.

Hope this helps to debunk the niacin myth.