Mouth swab soon need advice/help

Okay so I’ve got a mouth swab test coming up for a job. I’m not in a position to “not use” for 3 or 4 days prior to the test, maybe 12 hrs at best. Does anybody have a tried and true method for passing? Also, the test will be in the office the morning I start, but I’m not sure if it will be like right when I walk in or if it’ll be 15-20 min after I get there. Any info on experiences, passes and fails both would be greatly appreciated