Can a Drug Test Lead to a False Positive?

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on Nov 8, 2023.

A drug test is a laboratory test usually done to detect drug or alcohol use. Screening may be done for illegal drugs or prescription medicines, alcohol or other substances. Tests are usually performed on urine samples, but other types of specimens may also be used, such as hair, saliva, sweat or blood.

Any workplace drug-testing program, Federal or non-federal, should comply with required local, state, and federal laws.

Who gets a drug test?

Screening drug tests are often required for:

• pre-employment in the private and government sector
• athletes
• students
• healthcare workers
• probation or criminal activity
• opioid avoidance testing in the clinic
• vehicle or workplace accidents.

Not all jobs require drug testing. But according to some employer policies, drug screening may occur for pre-employment drug testing, random drug testing at a worksite, during an employee physical exam, and for any reasonable suspicion of drug use.

Even though these tests are common, people often have questions about the possibility of false-positive results. Anyone subject to a drug screen should confirm that secondary confirmatory testing (such as gas chromatography–mass spectrometry [GC-MS] or high performance liquid chromatography) is performed on any positive result received by the lab doing your testing.^1,2

Which drugs are tested in a drug test?

The drugs most commonly screened for in a drug test include¹⁶:

• Amphetamines / methamphetamine
• Barbiturates
• Benzodiazepines, like diazepam (Valium) or alprazolam (Xanax)
• Cocaine
• Marijuana metabolite / THC / Cannabinoids
• Opiates / Methadone / Heroin
• Phencyclidine (PCP)
• Prescription opioids like hydrocodone, oxycodone, methadone

Additional categories, as noted by SAMHSA, may include:

• Designer drugs like Bath Salts, Spice, or Ecstasy (MDMA, MDA analogues)
• Anabolic steroids
• Heavy alcohol use, or suspected use at the workplace involving an accident
• Other hallucinogenics (LSD, peyote, mescaline, psilocybin)
• Inhalants

Consequences of false-positives

Negative consequences of false-positive drug testing can include:²

• loss of employment
• jail time
• exclusion from competitive sports
• loss of privileges in a probation setting
• inappropriate medical care.

Even though drug tests are common, people often have questions about the possibility of false-positive result. Drug screening results can negatively affect many circumstances; therefore, accuracy is of the utmost importance.

Some data suggests 5% to 10% of all drug tests may result in false positives and 10% to 15% may yield false negatives.¹ Confirm with the lab you visit that a second, confirmatory test will be performed on any positive drug test that may occur. The results of your drug testing should remain confidential and kept separate from the regular employee work file.

How are drug tests done?

The initial test (called an immunoassay) allows for quick, large scale screenings with minimal cost. When urine or other specimens are collected, they are split into two samples, and one portion should be saved for confirmatory testing. 

When initial screening drugs tests (called immunoassays) result in positive results, a second confirmatory (Gas Chromatography Mass Spectrometry or GC-MS) test should always be done. GC-MS is a very specific test to identify separate compounds within a sample and is considered the “gold-standard” for drug testing. Confirmatory testing lessens the chance of a false positive, almost reducing the risk to zero.³ However, GS-MS may lead to a false negative if the GC-MS column is not designed to identify all potential compounds.¹

Rarely, a drug test may report the presence of illicit or prescription drugs in the immunoassay, although the person has not used these drugs. It is important that if you are undergoing drug testing, that you give a complete and accurate history of all prescription drugs, over-the-counter (OTC) medications, vitamin, dietary supplement, and herbal products used prior to the time of the sample collection. Certified labs are used for testing to ensure quality results.

Which drugs cause a false-positive drug test?

False-positive drug test results have been reported in multiple drug classes. Some of the more common classes include:

• Antihistamines
• Antidepressants
• Antibiotics
• Analgesics
• Antipsychotics
• Decongestants
• Certain over-the-counter (OTC) drugs

The chart below identifies common substances reported to cause a false-positive result based on the initial immunoassay testing. Any positive drug test should always be confirmed with a secondary test such as GC-MS that detects and provides identification and levels of a specific compound.

Learn More about Drug Testing: Drug Testing FAQs

Table 1: Drugs Reported to Cause a False Positive Urine Drug Screen

*Note: Table 1 does not cover all substances that may be responsible for false positive results. The above products may also be found in combination drugs. Results may be assay-dependent, and assays may be reformulated at any time to prevent a false outcome.

Additional Details

Can poppy seeds cause a false-positive drug test?

Poppy seeds, often found on bagels, rolls, and pastries have long been used as a defense against positive opiate test results encountered on a urine drug screen. It is known that poppy seeds do contain opiates - specifically morphine and codeine; however, content varies greatly depending upon seed source and processing. Food processing may lower the opiate levels in poppy seeds.

In a study published in the Journal of Analytical Toxicology,¹⁰ researchers determined opiate concentrations in opiate-free volunteers from 15 minutes to 20 hours after consumption of raw poppy seeds (15 grams) and prepared poppy seeds on a roll. Concentrations cutoffs of both 300 and 2,000 ng/mL were evaluated in urine samples. Oral fluid concentration cutoffs are set at 30 ng/mL.

• Using the 2,000 ng/mL cutoff for urine testing, morphine was not detected in urine from seeds on rolls. Raw poppy seed consumption (found to be unpalatable by participants) resulted in a detectable level in urine during the first 6 hours after consumption. At 20 hours, no morphine was detected for seeds on rolls or raw seeds in urine testing.
• With the 300 ng/mL cutoff for urine testing, morphine was detected after roll consumption in 50% of samples at 20 hours. After raw poppy consumption, morphine was detected in 100% of samples after 20 hours. With oral fluid testing, morphine fell below the 30 ng/mL detection limit with rolls after 30 minutes and from raw seeds at one hour.

In the lab environment, oral testing and the higher cutoff level of 2,000 ng/mL may help to reduce the number of false positives due to poppy seed consumption. The public typically eats rolls or pastries, and not large amounts of unpalatable raw poppy seeds where morphine levels are detected even after 20 hours at the 300 ng/mL limit.¹⁰

Federal authorities who mandate drug testing for federal employees have raised the required morphine cutoff concentration from 300 to 2,000 ng/mL (in urine) to reduce the number of opiate false positives due to poppy seed consumption. This level is current as of 2023, the most recent update as reported by The U.S. Department of Transportation.^15,17

Dextromethorphan (Tussin)

Over-the-counter cough and cold products such as the cough suppressant dextromethorphan have been reported to lead to a false positive result for phencyclidine (PCP, angel dust) and possibly opiates (narcotics).

Dextromethorphan is chemically related to levorphanol, a narcotic analgesic. The body metabolizes codeine to morphine and both substances may be found upon testing. Confirmatory testing can distinguish between the products.

Efavirenz (Sustiva)

Efavirenz does not bind to cannabinoid receptors. However, as noted in the efavirenz (Sustiva) package insert, false-positive tests for cannabinoids (THC) have been reported with some screening assays. Confirmation of positive screening tests for cannabinoids by a more specific method is recommended.¹¹

NSAIDs (ibuprofen, naproxen)

NSAIDs, such as ibuprofen, have often been implicated in causing false positives but most specimens are corrected with an alternate screening methodology if this is of concern. NSAIDs such as ibuprofen and naproxen are easily available over-the-counter in the US and are widely used.

In one report, authors concluded that acute or chronic use of ibuprofen or chronic use of naproxen were not regularly associated with false-positives but they do recommend secondary testing if needed.

Sertraline (Zoloft)

According to one report, doses of sertraline (Zoloft) exceeding 150 mg/day could lead to false positive benzodiazepine urine drug screens. Nasky and colleagues¹² reviewed 522 positive results for drug screens for benzodiazepines while taking sertraline, but were then negative with confirmatory GC-MS testing. They found 26 of 98 records to be identified as false positives.

The package insert for sertraline now warns about possible false positive tests for benzodiazepines under the "Warnings and Precautions" section. Confirmatory tests, such as gas chromatography/mass spectrometry (GC-MS), will help to identify sertraline from benzodiazepines with a suspected false positive result.

Lidocaine

Benzoylecgonine is the main metabolite metabolite of cocaine, and and is very specific for cocaine on a drug screen.^1,2  Some people may say they have used procaine or lidocaine as a way to defend cocaine abuse, but the presence of benzoylecgonine is specific for cocaine or coca tea.

As previously mentioned, confirmatory testing with GC-MS will identify individual drugs or metabolites in a sample, and almost eliminate the chance for a false positive result.

Passive marijuana smoke

The argument of inhalation of "passive" pot smoke from being in a room with people smoking marijuana (cannabis) is not a valid excuse, as it has been reported that the cut-off concentrations for lab analysis (usually 50 ng/mL) are set well above that which might occur from passive inhalation.

False Negatives

While it is difficult to tamper with a specimen, attempts have been made to alter samples, most often urine. Abnormalities in the urine screen may indicate that results may be a false negative or that there was deliberate adulteration of the sample.

Adulterants

A low creatinine lab value can indicate that a urine sample was tampered with - either the subject diluted their urine by consuming excessive water just prior to testing, or water was added to the urine sample. Creatinine levels are often used in conjunction with specific gravity to determine if samples have been diluted.

To help avoid this problem, the testing lab may color the water in their toilet to prevent the sample from being diluted with toilet water. The temperature of urine is also measured, to help prevent use of a substituted urine sample brought in from the outside.

Those attempting to foil the drug screen process may try to add certain enzymes to the urine sample to affect stability, but this often changes the pH, which is also tested. The most common adulterants used include certain oxidizing agents – such as nitrites, glutaraldehyde, chromates and halogens like bleach and iodine.

There have also been reports of people using products such as Visine eye drops to adulterate urine samples to cause a false-negative result for THC from marijuana. Ingredients in Visine, such as the preservative benzalkonium chloride or the borate buffer may decrease the concentration of 9-carboxy-THC in the initial urine immunoassay; however, upon confirmatory testing with GC-MS this metabolite will still be detected.²

Testing for adulterants in addition to drugs may be requested by the person requiring the drug screen.

Have Questions?

To join others who may have questions, concerns or want to voice opinions about illicit and prescription drug testing, false positives, false negatives, and other related issues, visit:

• Drug Testing Support Group

See also

• Anabolic Steroids - Abuse, Side Effects and Safety
• Blood Doping: Lance Armstrong & Pro Cycling
• Drug Testing FAQs
• The Risks for Teenagers of Using Steroids
• Toxicology Drug Testing

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Medical Disclaimer