Last updated: 25 July 2023 & medically reviewed by Dr. Jenni Jacobsen
A false positive drug test result occurs when a sample has been contaminated by a substance that has similar chemical properties to a drug that has been tested for. Read below to learn more about different causes of false positive results and why they occur as well as what to do if you get one on a drug test.
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What is a false positive drug test result?
Immunoassay drug tests are easy to use and readily available but are not 100% accurate. Inaccurate results can occur for a variety of reasons, including how the result is stored, processed in a lab, and whether it has come into contact with a contaminant. This may lead to what is known as a false positive result, where substances tested for appear in an immunoassay result despite not being in the system.
The most common cause of false positive results tends to be from foods, supplements, and other medications being present in the system that interfere with results or show the same metabolites of the drugs that are being tested for.
Potential causes of false positive results
While there are many instances that may lead to a false positive result, there are certain situations and products that are more common causes. Here are some of the top causes of false positive drug test results.
There are several types of antipsychotic medications that can cause false positive results in drug tests. For instance, quetiapine which is used for schizophrenia and bipolar can show up as methadone in urine tests, and chlorpromazine can show up as amphetamines.
Other psychiatric medications
The following medication may also cause a false positive result in standard drug tests:
Over-the-counter antihistamines designed to alleviate coughs, such as Benadryl, contain diphenhydramine. Despite its innocent nature, diphenhydramine may show up on drug tests as opioids, methadone, or even PCP.
Antiretroviral drugs like efavirenz (Sustiva) are used to treat HIV infections. If you are using a home drug test kit and are also taking this medication, it may show that you have marijuana metabolites (cannabinoids) in your system. Lab drug tests are more sensitive and will be able to correctly distinguish the two substances.
Nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen and naproxen, have been shown to cause false positive results for barbiturates and cannabinoids. Ibuprofen has also been shown to cause false positive results for PCP in some immunoassays.
Weight loss pills
Many forms of diet pills, such as Phentermine, contain chemicals similar to amphetamines that suppress appetite. Diet pills may flag on a drug test screening for amphetamines such as meth, which could lead to a second test being required.
Secondhand Marijuana Smoke
THC is the chemical in marijuana that creates the high people feel when it is taken. If you are with someone who is smoking marijuana, inhaling it can cause THC to appear in your system even if you haven’t directly taken it. However, it would require a large amount of secondhand marijuana smoke to yield a false positive result, as most cut-off points will account for this.
The small black seeds commonly found on bagels and other food products are from the poppy plant, the same plant from which opioids such as codeine and morphine are derived. More modern drug screenings will often have a high enough cut-off point to avoid poppy seeds causing false positives, though some labs may still use older tests.
While screening for alcohol in drug tests is relatively uncommon, certain ethanol-based products may cause a false positive if used regularly. Ethanol products that can affect alcohol tests include mouthwash, liquid medications, vanilla extract, or even hand sanitizers.
CBD (cannabidiol) products have boomed in popularity in the last decade, with CBD drinks, oil, chocolate, and other edibles being widely available. As CBD is derived from hemp, a relative of the cannabis plant, some companies add trace amounts (up to 5%) of THC to their products to boost their relaxing effects. This can cause some urine drug tests to show traces of THC, though the cut-off point for most modern drug tests will not cause a false positive for CBD.
Coca tea is a popular health remedy in Peru and other parts of South America. It is made from the same leaf as cocaine and contains the same metabolites. Coca leaves in any form are illegal in the US, though it may show up in drug tests if a person has consumed coca tea while south of the border.
What happens if you get a false positive test result?
It is advised that you disclose all substances you have consumed that may cause a false positive before taking the test to avoid incorrect results. If you still get a positive result but are certain there are no detectable substances in your system, then you should request a re-test.
If you request a re-test, then a lab technician will use your original sample and subject it to more rigorous testing. These tests will be able to separate the substances you believe have caused the false positive result from the test, making the final result more accurate for illicit substances.
Related: Types of drug tests