A method for confirming the active intake of marijuana and its active component .DELTA..sup.9-tetrahydrocannabinol (".DELTA..sup.9-THC") by detecting the amount of 11-nor-.DELTA..sup.9-THC carboxylic acid ("THCA") in oral fluid at the picogram per milliliter (pg/ml) level using chromatography/mass spectrometry/mass spectrometry ("GC/MS/MS").