A drug test is a screening process designed to detect the presence of illicit substances or their breakdown products, known as metabolites, in a biological sample. While these screenings are generally reliable, certain foods can introduce compounds that mimic drug metabolites, leading to a “false positive” result. A false positive occurs when the test indicates the presence of a target drug, but only a harmless, dietary compound was consumed. Understanding the specific foods and the underlying biochemistry is important for anyone facing mandatory testing.
How Screening Tests Are Fooled
The initial drug screening used in most settings is typically an immunoassay (IA), a rapid, cost-effective method. This test relies on antibodies designed to bind to a specific drug or its primary metabolite in the sample. If the target compound is present above a set cutoff level, the antibodies bind, triggering a chemical reaction that flags the sample as “presumptive positive.”
The issue of a false positive arises from a limitation of this technology called cross-reactivity. This occurs when a non-target compound in the urine, such as a molecule derived from a food item, shares a similar molecular structure with the drug metabolite the antibody is designed to detect. The antibody mistakes the dietary compound for the drug metabolite and binds to it.
The structural similarity is close enough that the immunoassay antibody cannot differentiate between the two compounds, resulting in a positive screen. Immunoassay tests are designed to be highly sensitive and detect a broad class of drugs, but they sacrifice some specificity, making them susceptible to these chemical look-alikes. This preliminary result is never considered final and requires further investigation.
Specific Foods That Mimic Drug Compounds
Poppy Seeds
The most known example of a food causing a false positive is the consumption of poppy seeds, which can trigger a positive result for opiates, specifically morphine and codeine. Poppy seeds are harvested from the opium poppy plant (Papaver somniferum). While the seeds themselves do not contain active drugs, they can become contaminated with the plant’s milky sap during processing. Trace amounts of opiate alkaloids like morphine and codeine can remain on the seeds, leading to detectable levels in urine shortly after eating items like a bagel or muffin.
Hemp Products
Another common dietary source of false positives is the use of hemp seed products, which can register as positive for Tetrahydrocannabinol (THC) or its metabolites. Although hemp is legally distinct from marijuana due to its low concentration of Delta-9-THC (typically less than 0.3%), products such as hemp seeds, hemp seed oil, or full-spectrum Cannabidiol (CBD) products can contain trace cannabinoids. Consistent or heavy consumption of these products may introduce enough THC metabolite into the system to exceed the initial screening cutoff level for cannabis.
Coca Leaf Tea
A geographically specific culprit is coca leaf tea, or mate de coca, a traditional beverage consumed in South America. Since the tea is brewed directly from the leaves of the coca plant, it naturally contains small amounts of the coca alkaloid and its metabolite, benzoylecgonine. Drinking just one cup of this tea can lead to a positive screen for cocaine metabolites for up to 36 hours.
Medications
Beyond these foods, certain over-the-counter medications can also interfere. High doses of non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen or naproxen have been reported to cause rare cross-reactivity with the immunoassay tests. The chemical structure of these medications is close enough to the target drug classes to occasionally confuse the initial screening antibodies for barbiturates or even cannabinoids.
What To Do After A Positive Screen
The first step following a presumptive positive result from an initial immunoassay screening is to request a confirmation test. A confirmed positive result requires a different and highly precise analytical method, typically Gas Chromatography-Mass Spectrometry (GC-MS).
GC-MS first separates the various compounds in the sample using gas chromatography. It then identifies each substance based on its unique molecular fingerprint using mass spectrometry.
GC-MS is often referred to as the “gold standard” of toxicology testing because it offers superior specificity and accuracy. Unlike the less specific immunoassay, GC-MS can definitively distinguish between a dietary compound and a synthetic drug metabolite. If the substance is confirmed to be food-derived, the test result must be reported as negative.
The process involves a Medical Review Officer (MRO), a licensed physician who is an impartial gatekeeper of the testing process. The MRO confidentially contacts the individual who screened positive to discuss any legitimate medical or dietary explanations for the result. Providing these details allows the MRO to interpret the GC-MS results accurately and ensure the final report is correct.