Here is an excerpt I found on "false positive" marijuana test...which don't exist. No commonly used substance is known to produce a false positive for marijuana. One exception is a prescription drug for ulcers known as Protonix®, which has been reported to cause a false positive for THC on immunoassay tests. Other prescription and over-the-counter medicines can cause false positives for amphetamine and other illicit drugs on immunoassay tests. However, all of these legal drugs can be distinguished