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There are hundreds of over-the-counter and legally prescribed drugs that will cause a drug testing false positive. Standard drug tests fail to distinguish between legal and illegal substances.

The EMIT test can be wrong 25% to 30% of the time, and can result in drug test false positives from legal substances up to 40% of the time.

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Drug Testing - False Positives

Drug Testing Advice | False Positives

In drug testing, a false positive is defined as a drug free sample falsely being reported as showing positive for drugs. This can occur for a number of reasons including: improper laboratory procedure, mixing up samples, incorrect paperwork and passive inhalation. But the most common cause of drug testing false positives are cross reactants. A cross reactant is a substance which because of its similar chemical structure to a drug or its metabolite can cause a false positive result.

The following substances can cause cross reactivity on an Immunoassay screen but are unlikely to be mistaken on a Gas Chromatography/Mass Spectrometry test (for more information on these tests and others, visit the types of test page):


Ibuprofen is a common pain reliever and anti-inflammatory which even in low doses used to cause a false positive for marijuana/cannabis on the EMIT test. The EMIT has been changed to use a different enzyme to eliminate these drug test false positives. But recent evidence suggests that Ibuprofen taken in very high doses, along with other anti-inflammatories such as Naproxen will still interfere with the EMIT test.

Decongestants and Cold Remedies

Phenylpropanolamine and ephedrine are both substances found in many over-the-counter cold remedies. They can result in a drug test false positive for amphetamines on the EMIT test. Antitussives, to suppress coughs, such as dextromethorphan and perylamine may cause a drug test false positive for opiates.


Aside from when this class of drugs is specifically tested for, some of them including amitriptyline can test positive for opiates for up to three days after use. Even quinine in tonic water can also cause a positive result for opiates.

Poppy Seeds

Poppy seeds which are usually found on bread contain traces of morphine and can lead to positives for opiates. Codeine, which is found in many pain relievers, may cause a false positive for morphine or heroin because of its similar chemical structure.


Certain newly developed antibiotics including amoxicillin and ampicillin have been reported to cause false positives for cocaine.


This treatment developed for use by AIDS patients will cause a false positive for anabolic steroid use.


Diazepam may cause a false positive for PCP.


A small fraction of the population excrete large amounts of certain enzymes in their urine which may produce a positive drug test. The enzymes in question are endogenous lysozyme and malate dehydrogenate, which according to research may run as high as 10% of positive samples.


Melanin is the pigment which protects skin and hair from UV light. It is also very similar in chemical structure to THC (Tetrahydrocannabinol, the active component in cannabis) and some data exists claiming it causes false positives for cannabis. Unfortunately, an equal amount of data suggests that there is no link whatsoever.

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