There are several prescription drugs on the market that can cause false-positive urine tests on a drug screen. And several are employed to detect substances in a person's urine. Some of these drugs show up as different substance than what was actually ingested by the person being tested. However, none of them are infallible according to the organisation NORML. In fact, litigation challenging these urine tests have become a problem because of reliability issues.
Urine test methods
The EMIT, Enzyme Multiplied Immunoassay Technique, ELISA, RIA (Radioimmunoassay), GC/MS (Gas Chromatography/Mass Spectrometry), TLC (Thin Layer Chromatography), and GLC (Gas-Liquid Chromatography) tests are the standard methods used to detect drugs in urine. The EMIT, ELISA, and RIA are the most commonly used.
The EMIT test doesn't require medical personnel to administer, and is the cheapest. Immunoassay tests like the EMIT, ELISA and RIA are screening tests, and require a confirmatory test by a more sensitive diagnostic equipment like the GC/MS test to back up the results. GC/MS is similar to TLC, and GLC, and are expensive to use, plus they are laboratory based, unlike the EMIT test which can be used in the field.
False positives and false negatives
False-positives are a problem in drug test screens, that is why confirmatory tests have to be done. What are false-positive tests? "False-positives are...where metabolites of a legal prescription drug, or other substance such as poppy seeds, tests positive for the metabolite of an illicit drug," according to NORML. Metabolites are the elements in urine the EMIT test identifies in rendering its verdict. Along with false-positive results, there are false-negative ones as well, but those come from the more advanced forms of drug screening like with the GC/MS test.
According to NORML, there are no known substances that "...produce a false positive for marijuana." Before urine tests were refined, some over the counter, and prescription drugs would cause a false-positive test result for THC. Drugs like ibuprofen contained in Advil, Motrin, and Nuprin, would confound the drug tests. However, that is not the case any longer, at least according to NORML.
There is one prescription drug that will cause a positive urine test for THC, and that is dronabinol (Marinol). This drug is used for anorexia, HIV patients who have lost excessive weight, and cancer patients. Other prescription drugs will cause false-positive urine test results, but not for THC.
Companies on the Internet claim to market substances able to mask urine test results if they are dirty for illicit drugs. If you flush your system with water, it will alter the creatinine levels, and colour of your urine indicating to the testing facility that this is exactly what you did. And many of these so-called detoxing, and urine cleansing companies tell you to stop taking any drugs 1 to 3 days prior to the urine test before you take their cleansing product. Most drugs are out of your system in anywhere from 12 hours to 4 days. Be cautious of these companies.
Passive inhalation of marijuana smoke has produced positive THC urine tests. However, the cutoff points that are now used in the tests are designed to allow for those instances. If you do have to submit to a urine test for an employer, or for some other reason, write down a list of all the medications you are taking, including any non-prescriptions drugs, and herbal supplements.
When you do take the test, and it comes up positive for THC, take steps to have your own confirmatory test done so you have some proof to fight the initial urine screening result. Better to get confirmation from an outside source. Urine tests are not infallible, and you may be able to challenge a positive urine test.