I know that niacin will help you pass a pee test but how can you pass a test when they swab your mouth?
The obvious answer is, ...don't do drugs. But if you have done drugs, the best way to hide it is to eat a LOT of meat in the 24 hours prior to the test, combined with drinking a lot of water. This will throw the baselines off.
If they ask you questions about what you've eaten or drunk lately, make sure to say you had muffins (if they ask what kind, reply that they were kind of lemony with little seeds on top...you are trying to convince them you ate poppy seeds without knowing it could alter your test), and that you've been taking over the counter pain relievers but can't recall what exactly (was it execdrin, advil? oh hell I don't recall).
Drug metabolites don't stay in your mouth for very long. Each drug has a different time, but usally 72 hours after you consume, it is out of your saliva. If you do not have enough time, then there is a product on the market called Oral Clear which is good for masking drug metabolites in your saliva.
So what drugs you doing? lol
I would wait as long as I could before I took the test. I know a guy who put a bottle cap of bleach in a gallon jug and drank that. He passed his test. He was smoking weed not sure about other drugs.