What causes false possitive drug screen for cocaine my career depends on an answer?

Here is an excerpt I copied from an interesting article. I hope this helps.

False positives

Laboratory slip-ups are to blame for many of the mistaken test results. But even perfect labs running perfect tests will also find many false positives.

False positives are caused by:

*Nasal decongestants such as Dristan, Neosynephren, Vicks Nasal Spray, and Sudafed, which cause false positives for amphetamines.

*Poppy seeds like those on a dinner roll, which cause false positives for opiates. Even the highest-quality test currently available, the GCMS test, still confuses poppy seeds with heroin.

*Pain relievers such as Advil, Nuprin, Midol, Trendar, or any medicine containing Ibuprofin cause false positives for marijuana.

*Antibiotics such as amoxicillin or ampicillin cause false positives for cocaine.

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The above list is only a beginning. Most substances which people have in their blood have never been tested to see if they cause false positives. Indeed, even some natural body enzymes cause false positives. According to research validated by the Congressional Office of Technology Assessment, melanin, the natural pigment which makes skin dark, can cause false positives for marijuana.