The saliva test is actually the most "lenient" of all the drug tests. Basically, when you smoke, say, marijuana, many of the particles you breathe in will contact your saliva, and some of them will "stick" to it. So, when the swab picks up some saliva, it will also 'grab' some of the smoke particles that landed in your saliva, and if such narcotics are detected in a high enough *concentration*, then the test will return positive. For some opiates and cocaine, some of the particles from the blood will enter into the saliva, making for a longer test period, of maybe 2-3 days at most. For anything smoked, the test is only good for 12-16 hours tops. IIRC, it is one of the cheaper tests, and results come back quickly.