as far as I know, the only perscription or OTC drug that can caus false +'s for coke is
Amoxicillin (an antibiotic).
cocaine and similar topical anesthetics are also sometimes used in sugery, and I suppose even a topical could get into your bloodstream. anyway, coke is only in yr system for a few days after single use, so even if you DID use coke, it shpould be out of your system in 3-4 days tops, so it sounds like the lab may have fuked up (it does happen) or yr lying (not to be accusatory but that happens too). Raise hell and demand a retest, esp if yr freedom depends on it.
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