What can cause false-positive drug tests?

False-positive drug test results, where a test wrongly suggests illegal drug use, can be caused by a range of everyday substances. These include over-the-counter medications, prescription drugs, certain foods like poppy seeds, and even environmental exposure.

In this guide, we explain what can cause false-positive drug test results.

What is drug testing?

Drug testing is a scientific process used to detect the presence of drugs and their metabolites (breakdown products) in the body.

When a person consumes drugs, a proportion of the drug and its metabolites are released into the bloodstream and excreted from the body in a variety of ways, including in hair, nails, urine and saliva (oral fluid).

These samples can then be tested for the presence of drugs, including illegal drugs and prescription medications.

Drug testing is used in a variety of settings to determine if someone has used drugs recently or over a longer period (historic/past drug use), including:

• for personal reasons (peace of mind)
• for private official matters (e.g. child custody/divorce proceedings, contractors and consultants who require a drug test to start work)
• in legal settings (e.g. child welfare and protection matters)
• in the workplace (in line with workplace drug testing policy)

Explore our range of drug testing services

Types of drugs tests and how they work

There are several types of drug tests, each with different uses and detection windows.

Hair drug testing

Hair drug testing – also called hair strand testing – is often considered the gold standard. The test works by analysing a cut hair sample for the presence of drugs and their metabolites. When a person consumes drugs, a proportion of the drug and its metabolites travels through the bloodstream and enters the hair follicle, becoming trapped in the innermost layer of the hair shaft.

At AlphaBiolabs, we can analyse up to 12 cm of scalp hair, which represents up to 12 months of possible drug history, based on average hair growth.

Nail drug testing

Nail drug testing works by analysing fingernail clippings or toenail clippings for the presence of drugs and their metabolites.

When a person consumes drugs, they are metabolised (broken down) by the liver. Some of the drug and its metabolites enter the bloodstream and can pass from the blood vessels below the nail bed, into the keratin fibres along the entire length of the nail. This makes it possible to detect drugs in the nails during analysis.

Fingernails provide an overview of up to approximately six months for historic drug use, while toenails provide an overview of up to approximately twelve months. This is because fingernails and toenails grow at different rates.

Oral fluid (saliva) drug testing

Oral fluid testing can be used to detect very recent drug use (up to 48 hours after drugs were consumed).

The test works by analysing a saliva sample for the presence of drugs and their metabolites. It’s also easy to perform and very difficult to cheat, making it ideal for on-the-spot drug testing (e.g. for child welfare and protection matters, or as part of a workplace testing programme).

Urine drug testing

Urine testing is one of the most common methods for detecting recent drug use and works by analysing a urine sample for the presence of drugs and their metabolites.

Like oral fluid testing, it provides a narrow window of detection (up to 4 days after drugs were consumed).

Learn more: How do you prove abstinence from drugs?

What is a false-positive drug test?

A false-positive drug test happens when the result incorrectly shows that a person has taken a drug, even when they haven’t.

This can happen for a variety of reasons, such as when a substance in the body is chemically similar to the drug being tested for, or because of environmental exposure.

Fortunately, false positives are rare, and accredited laboratories like AlphaBiolabs follow strict procedures to ensure accuracy and significantly reduce the risk of false positives.

What can cause false-positive drug tests?

Although very rare, false-positive drug test results can be triggered by a range of everyday substances, including medications, certain foods, and even environmental exposure (e.g. in hair and nails).

However, these rare instances of false-positive results are typically limited to on-the-spot oral fluid and urine screening tests, which are sometimes performed to assess recent drug consumption.

In contrast, hair, nail, oral fluid, and urine tests performed in the laboratory using confirmatory techniques (such as LC-MS/MS) are highly specific and are not subject to the same rare occurrences of false positives as initial screening tests.

Thankfully, reputable testing laboratories like AlphaBiolabs have numerous processes and procedures in place to significantly reduce the likelihood of false-positive drug test results, including:

• Only sourcing point-of-care screening devices from reputable manufacturers whose products are widely known to provide accurate, reliable results during point-of-care tests
• Using confirmatory laboratory testing methods to ensure that drugs detected are present in the sample due to being consumed, rather than as a result of passive/environmental exposure. Passive exposure is unlikely to cause a positive result, but it is considered during interpretation, especially when levels are close to cut-off levels or when no metabolites are detected (produced by the body when substances are broken down following ingestion)
• Washing all hair and nail samples before testing

Even with this in mind, it is still helpful to know what can cause a false-positive drug test result.  If you are due to take a drug test, it is important that you disclose any over-the-counter medications, prescription medications, food, drink, or environmental exposure to drugs that could potentially impact the results of the test.   

Quick reference: Common substances that have the potential to cause false positives during point-of-care oral fluid or urine screening tests:

Substance/Medication	Rare but may cause false positive for
Ibuprofen, naproxen	THC, PCP, barbiturates
Diphenhydramine (antihistamine)	PCP, methadone
Dextromethorphan (found in some cough syrups)	Opioids, PCP
Poppy seeds	Opiates (morphine, codeine)
Methylphenidate (Ritalin)	Amphetamines
Omeprazole/Proton Pump Inhibitors (PPIs)	THC
Sertraline (Lustral)	Benzodiazepines
Phentermine (sometimes used in weight loss medications)	Amphetamines

Over-the-counter medications that can cause false-positive drug tests

Some over-the-counter (OTC) medications (available without a prescription) have the potential to trigger a false-positive result during a point-of-care oral fluid or urine screening test. Examples include:

• Ibuprofen (sold under brands like Nurofen or as a generic) and naproxen (brand names Naprosyn, Synflex)

  These non-steroidal anti-inflammatory drugs (NSAIDs) can, on rare occasions, cause false positives for cannabis (THC), phencyclidine (PCP), and barbiturates.

• Antihistamines (e.g. Diphenhydramine)

  Antihistamines are used to treat allergies, colds, and sleep issues. One example, diphenhydramine – found in some night-time cold and flu remedies – has been reported to cause false positives for methadone and PCP.

• Cough syrups (containing Dextromethorphan)

  Dextromethorphan is a common ingredient in cough remedies. In high doses or with frequent use, it has been known to cause false positives for opioids and PCP.

While these situations are uncommon, they highlight why it’s important to disclose all medications – even non-prescription/over-the-counter medications – before a drug test.

Prescription medications that can cause false-positive drug tests

Some prescription medications have the potential to cause false-positive drug results during point-of-care oral fluid or urine screening. This can happen when a medication contains a chemical compound that resembles an illicit drug, or when it’s metabolised in a similar way.

• Antidepressants

  Several antidepressants have been known to interfere with drug test results including:

  • Sertraline (Lustral) – can cause a false positive for benzodiazepines.
  • Fluoxetine (Prozac) – can lead to a false positive for amphetamines.
  • Trazodone – can cause a false positive for amphetamines.
  • Venlafaxine (Efexor) – has been associated with false positives for PCP.

• Antibiotics

  Although rare, certain antibiotics have been linked to false positives:

  • Rifampicin (used for tuberculosis) – may interfere with opioid testing.
  • Fluoroquinolones (e.g. ciprofloxacin, levofloxacin) – have been reported to cause false positives for amphetamines or opioids in isolated cases.

• ADHD medications

  Stimulants used to treat ADHD are closely related to amphetamines:

  • Methylphenidate (Ritalin, Concerta XL) – may cause a false positive for amphetamines.
  • Lisdexamfetamine (Elvanse) – breaks down into dextroamphetamine, which may result in a true positive for amphetamines*.
    *Also applicable to hair and nail testing

• Proton Pump Inhibitors (PPIs)

  These drugs are commonly prescribed in Ireland for acid reflux and stomach ulcers:

  • Omeprazole, pantoprazole, esomeprazole, and lansoprazole – have been known to cause false positives for THC, the psychoactive ingredient in cannabis.

• Weight loss medications

  Phentermine – although not licensed for use in Ireland – may be obtained through private slimming programmes and can trigger a false positive for amphetamines.

Related: What medications can cause a positive alcohol test?

Foods and drinks that can cause false-positive drug tests

As well as medications, certain foods and drinks also have the potential to cause a false-positive result during a point-of-care oral fluid or urine screening test. While these substances are perfectly legal, they can contain compounds that are chemically similar to drugs.

• Poppy seeds

  Found in foods like muffins, bagels, and certain pastries, poppy seeds are derived from the same plant used to manufacture opium.

  While the seeds themselves don’t contain any illegal drugs, they can contain trace amounts of morphine and codeine, which are opioids. As a result, consuming large amounts of poppy seeds could cause a false positive drug test result for opiates.

• Coca Tea (Mate de Coca)

  While not widely available in Ireland, coca tea is sometimes brought back from countries like Peru or Bolivia as a natural remedy or souvenir.

  Made from coca leaves, it contains alkaloids that can result in a true positive for cocaine. Even one cup of coca tea can lead to a detectable level of benzoylecgonine – a metabolite (breakdown product) of cocaine – in a urine sample.

  This is not a false positive but could be considered a false positive by the person taking the drug test, as they may not realise that the tea contains active cocaine derivatives.

• Hemp products

  Hemp-based foods such as hemp oil, seeds, or protein powders legally sold in Ireland must contain only trace levels of THC (tetrahydrocannabinol), the psychoactive compound in cannabis.

  However, heavy or frequent consumption of these products has, in rare instances, been associated with a positive drug test result for cannabis.

  While unlikely with moderate use and highly regulated products, it is still technically possible – especially with unregulated or imported supplements.

• Tonic water

  Tonic water contains quinine, a compound historically used to treat malaria. High levels of quinine have been reported to cause false positives for opiates. However, this is usually only a concern with excessive consumption, and trace amounts in tonic water are unlikely to trigger a false positive result in modern-day drug tests.

While foods and drinks rarely lead to false-positive drug test results, there are documented exceptions. To be safe, it’s best to inform the testing provider of any unusual foods, herbal teas or supplements you’ve recently consumed – especially if the test result could have legal or professional consequences.

Environmental causes of false-positive drug tests

While uncommon, certain environmental factors can lead to a ‘positive’ result for drug use, even if no drugs were taken intentionally.

These situations are rare, with reputable testing laboratories like AlphaBiolabs taking steps to reduce the impact of external contamination and help ensure that results are more likely to reflect active drug use (through consumption) rather than incidental or passive exposure.

Examples of environmental exposure include:

• Second-hand cannabis smoke

  Exposure to cannabis smoke – particularly in enclosed, poorly ventilated spaces – can result in tetrahydrocannabinol (THC) being deposited onto the surface of the hair.

  Studies have shown that non-users who spend time in these environments may have detectable levels of cannabinoids in their hair.

  However, during laboratory testing, these levels are clearly distinguishable from levels found in the hair of people who actively use (i.e. smoke) cannabis.

• Contact with contaminated surfaces

  Drugs or drug residues can sometimes be transferred onto the hair of a non-drug user from contaminated materials such as bedding, towels, hats or clothing – particularly if the person spends a lot of time with someone who uses drugs (e.g. sharing bedding or clothing).

  To guard against this, reputable testing laboratories like AlphaBiolabs routinely wash hair samples when they arrive at the laboratory, to reduce the presence of any external contaminants before the hair is analysed. This process is designed to help ensure that the results more accurately reflect drugs that have been used and absorbed into the hair through the bloodstream, rather than substances that may have been picked up from the environment, such as smoke or surface contact.

  In child welfare and protection cases, the solution/fluid used to wash the hair samples is also tested which can help determine whether the child has been exposed to second-hand drug while in the care of a parent or guardian.

How to reduce the risk of a false-positive drug test result

While false-positive drug test results are rare, there are some simple steps you can take to reduce the risk of a false positive if you’re preparing to take a drug test:

• Tell the testing provider about any medicines or supplements you might be taking (including prescription medications, over-the-counter medications, nutritional and sports supplements)
• Avoid high-risk foods and drinks such as poppy seeds and unregulated hemp or CBD products
• Steer clear of environments where drugs are used and avoid sharing items like bedding, towels and clothing with people who use drugs

Where can I get a drug test?

AlphaBiolabs has over 20 years’ experience providing accredited drug testing services for peace of mind/home use, the legal profession and the workplace.

We offer a wide variety of drug tests to suit all needs, including:

• Home Drug Testing Kit
• Drug testing for legal and official matters
• Drug testing for legal professionals, social workers and local authorities
• Drug testing for employers implementing workplace testing programmes

To find out which test is best for you, call our friendly, discreet Customer Services team on 01 402 9466 or email info@alphabiolabs.ie for more information.