It is possible for naproxen to cause false-positive results for barbiturates on a urine drug test, according to the College of Pharmacy at University of Illinois at Chicago. On rare occasions, naproxen also erroneously appears on a urine drug test as a chemical component of marijuana.
Urine drug screens often use immunoassays because they produce inexpensive and fast results, states the College of Pharmacy at University of Illinois at Chicago. The chemicals used in the test contain antibodies against both drugs of abuse and the substances they break down into, which are called metabolites. The most common drugs immunoassays look for are amphetamines, marijuana metabolites, cocaine metabolites, opiate metabolites and phencyclidine, or PCP.