CuraPhyte Technologies had to address this question for both its work with professional athletes and truckers that travel to the USA. The results obtained from a private testing laboratory revealed that their California poppy does not give false positives ('show up') on a drug screen.
California poppy contains alkaloids that are in the same family as codeine and morphine. However, a drug screen performed using mass spectrometry (MS) technology will be able to identify that these alkaloids are not banned drugs or illegal drugs. MS identifies the molecular structure and allows the lab to differentiate between two similar chemical structures.
False positives occur when a substance gives a positive result in a screening test but in fact is negative. Usually the action taken subsequent to a positive result is to perform the more sophisticated analysis that involves mass spectrometry (MS) (technology used is: Liquid Chromatography- Mass SPectrometry; refered to as LC-MS). The MS testing allows the laboratory to identify the actual molecule and this then tells them that the result is negative and there was a false positive.
I always recommend that people declare they are taking California poppy to their employer or sports testing agency. This way if a false positive occurs then the laboroatory will automatically perform the other tests (MS). It is also a good idea to give them the name of the manufacturer of the California poppy; this way they can call the company to obtain any informaiton needed.