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There are reports that amoxicillin may cause a false positive for cocaine on a screening drug test, although studies have not confirmed that it will truly do so. The studies have been done on a limited number of patients, but there clearly is not a substantial rate of false positives.
However, it is also true that we do not know the cause of many false positive results. The screening tests have a fair amount of false positives, even in people that are not taking any other medicines. The usual approach when a screening drug test is positive in someone not taking the drug is to then perform a confirmatory drug test, such as a gas chromatography/mass spectrometry, or GC/MS. If the confirmatory test does not show the drug, then that is proof that the screening test was a false positive.
At this point, rather than trying to find an explanation for a false positive, it would be better for her to insist that a confirmatory test be done.
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