What Can Cause a False Positive Drug Test?

The aim of pre-employment drug tests is to discover if a potential employee uses illicit substances. Companies are keen to screen future employees because if a habitual drug user is hired, it can impact productivity for the company as a whole.

The problem is that these drug tests are fallible. A false positive drug test is an unfortunate result that many non-drug users have found themselves receiving, which can be a frustrating and confusing experience.

In this article, we’ll explain what a false positive is, the reasons why it can occur, as well as look at the most common over-the-counter medications, foods, herbs, and supplements that can cause false positives.

Let’s begin with the most important question.

What is a false positive drug test?

In a nutshell, a false positive drug test result is one where the drug testing process shows that drug metabolites are in the system of someone who has not consumed any of the substances that are being screened for.

False positives can occur when any of the major drug testing procedures are used. They are, however, far more likely with UDS (urine drug screen) immunoassays:

Paper strips are dipped into the urine testing sample, and then, just like a pregnancy test, they will cause a colored line to appear that will display the result of the test.

Suppose a positive result shows up on immunoassay results. In that case, professional drug testing facilities will use a method called Gas Chromatography Mass Spectrometry (GC/MS) to ensure that the result is correct.

Gas chromatography or liquid chromatography confirmatory testing is a more intensive procedure that should then identify if result is a true positive or an error with the original test and therefore a false positive.

What can cause false positive results?

There are five main causes of false-positive results. Let’s have a look at each of them.

Mishandled samples

The first reason is quite simply that the sample was mishandled or compromised in the laboratory. Even though there are very strict procedures for how a sample should be handled, this doesn’t mean that lab workers can sometimes cut corners to try to get more samples analyzed in a day.

A small mistake can lead to big consequences when it comes to analyzing a drug test sample, and unfortunately, these mistakes are common.

The lab workers aren’t the only ones who can make mistakes. The collectors can also mislabel urine samples, or they might even forget to seal them properly.

If either of these things occurs, then the sample could be contaminated or compromised, resulting in false positives.

Accidental intoxication

Another reason for a false positive drug test result is not to do with the negligence of the people who handle or analyze your sample at the drug testing site.

Instead, it could quite simply be because the test subject has accidentally or unknowingly ingested drug toxins.

There are many ways that this can happen, but the most likely is that the test subject has been in the vicinity of marijuana smoke. THC has entered their bloodstream through inhaling second-hand smoke, which then appeared on a urine screening test.

This is even more likely if the testing facility has chosen to apply low cutoff levels when analyzing test results. When these levels are low, then it’s much easier for a person who has no history of drug use to end up producing a urine sample that receives a false-positive result.


The final reason for a false-positive result is the most common. It’s down to something that’s referred to as ‘cross-reactivity’. In layman’s terms, cross-reactivity is when an immunoassay screen mistakes one substance for another.

When urine drug tests show a positive, then the sample will need to be analyzed through GC/MS as this method will then be able to determine the exact chemical compound that has been detected in the urine sample.

This means that cross-reactivity can then be discovered to cause a false positive rather than presence of illegal drugs.

12 substances that may cause a false positive

There are many common substances found in everything from prescription medications to ingredients in everyday foods, herbs and supplements that may cause a false positive. Let’s take a look at twelve of the most prominent.

1. Poppy seeds 

If you’re a fan of Seinfeld, then you already know that poppy seeds can cause a false positive at a drug screening. A poppy seed can lead to a false-positive test result because it contains naturally occurring compounds also present in morphine and codeine.

It is rarer for a poppy seed to trigger a positive when the cutoff levels are high – 2,000 ng/l – but there are lots of testing facilities that have a much lower cutoff level for poppy seeds (opioids).

2. Vitamin B12

Vitamin B supplements are very popular these days. People take them as a way to boost energy, but they can also trigger false positive drug test results. The reason is that they can include riboflavin, which is often derived from hemp seed oil.

That means that trace amounts of THC (marijuana) can be present, meaning that the test results will wrongly suggest that you have been ingesting marijuana, as showed by a case report.

Check the ingredients on your Vitamin B12 supplements to see if riboflavin is an ingredient, as this can be the reason for your false-positive urine drug test result.

3. Kava

Kava is another dietary supplement that people take as sleep aids. It can be ingested in pill form or be drank as a liquid. There is anecdotal data that shows ingesting kava may result in false-positive results for MDMA.

4. Energy drinks

Many of us can’t make it through the working day without some liquid stimulation in the form of an energy drink. Strangely, many of the top energy drinks have been proven to cause false-positives for everything from methamphetamines to THC (marijuana).

Even opiates have shown up on drug tests of people who have drunk Red Bull in some instances1. It tends to be more common when the test subject has ingested an energy drink a short time before submitting to urine drug screening.

If you’ve consumed Red Bull, Bang, 5-Hour Energy, or even tonic water before drug tests, it may be a good idea to invest in home testing kits, so you can test yourself in advance.

5. Zantac (ranitidine)

In most drug screenings, amphetamine immunoassay is common. And it has been found that people who have Zantac prescriptions can end up receiving false positives on urine tests.

Why? It is because of cross-reactivity. The chemical compound of these acid reflux prescriptions resembles that of amphetamines.

6. Atarax (hydroxyzine hydrochloride) 

Atarax is commonly taken to aid anxiety, as well as to reduce itchiness. It is the 99th most commonly prescribed medication in the USA. If you take it regularly, then it can cause a false-positive drug test result for benzos.

7. DXM (dextromethorphan) 

DXM is found in many over-the-counter drugs that are used for stifling coughing. It’s also been known to cause false-positives for drugs like PCP and opiates. However, it is only common when very low cut off levels are used in the testing process2.

8. Robaxin (methocarbamol)

Robaxin is the most common brand name for the chemical compound methocarbamol. It is a muscle relaxant. And there is anecdotal data that suggests that people have received false-positive test results for opiates and benzos when taking Robaxin.

9. Ritalin (methylphenidate)

Methylphenidate is found in prescription drugs like Ritalin, which are used to treat ADHD and narcolepsy. There have been many reports of Ritalin leading to positive urine drug screens for amphetamines, as well as methamphetamine.

In rare cases, Ritalin has even caused false-positive drug test results for LSD.

10. Dental anesthesia

If you undergo a major dental procedure, you will be given various pain medications or drugs. There is a wide range of substances used for this purpose, and some can can cause false positive tests.

For example, amoxicillin has been known to cause false-positive test results for cocaine. Other sedative medications can also lead to false-positive outcomes for opiates and amphetamines.

11. NyQuil

Many Americans have turned to NyQuil for relief when suffering from a cold. But NyQuil has also been reported to cause false positives for MDMA drugs.

Similar cold medications like Advil have also led to THC (marijuana) to show up on a drug tests. That’s also the case with any NSAIDs that include ibuprofen as an ingredient.

12. Midol 

Midol is one of the most popular medications that’s used to treat menstrual cramping. Like Advil – and many pain-relief medications – it has ibuprofen as an active ingredient. So, taking Midol may result in false positive tests for marijuana use.

How to dispute a false positive result

If you’re certain that you’ve received a positive drug test result wrongly, then your first course of action should be to demand that further testing is done. As we’ve already learned, there are many reasons why you might have been given a false-positive.

If you know for certain that drug abuse is not the cause of the false-positive test results, then you can make it clear that you’re willing to submit to further testing.

A blood test or a hair follicle test will prove that the outcome was incorrect and that no illegal drugs are in your system. Urine drug testing should always be confirmed via GC-MS or HPLC test.

If further drug testing still reveals positive results, then it’s time to have a chat with your employer. You may want to explain that you are on certain medications or prescription drugs that could have caused a false-negative.

We’ve covered pre-employment drug tests in great detail in another article.

What is a false negative drug test?

False negatives are the opposite of false positives. In a nutshell, if you receive a false negative result, then it means that drug testing has not detected drugs in your system when you know that you do have illegal substances in your system.

The most common cause for false-negative drug test results is when urine has been tampered with so that no drugs show up in the sample. This can either occur by dilution of the urine, or its adulteration with products like eye drops.

The second reason why a drug test could result in a false-negative is down to the testing methods used. If the cutoff levels are high, then trace amounts won’t cause a positive drug test result.

For example, the standard cutoff level for PCP on a regular urine drug test will be 25 ng/ml. That means that if you take PCP within 14 days of a drug test, then you will likely test positive depending on the testing methods used.

Suppose the testing facility chooses to put the cutoff level higher for PCP. In that case, false negative can occur because the body has processed the substance to a level below the cutoff point.


Drug testing is a common procedure that can lead to surprising results. That’s because drug tests – especially panel drug tests – can misinterpret legal drugs or medications for illegal drug metabolites like THC-COOH (tetrahydrocannabinol).

Everything from poppy seeds to pain management, anti-inflammatory or weight loss pills like Phentermine, as well as over-the-counter NyQuil cough suppressants has been reported to cause false-positive results.

Ethanol-containing breath cleaning products and vanilla extract can result in testing positive for alcohol.

It would help if you considered what you have eaten, drank, or taken recently, as it’s likely that this is the cause of the false-positive. You should also request further testing, as more in-depth analysis will usually reveal that you should’ve received a negative result.