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Can a Drug Test Lead to a False Positive?

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on Aug 8, 2021.

Have you been asked to take a drug test? Maybe this caught you off guard. Screening drug tests are often required for pre-employment in the private and government sector, for athletes, students, healthcare workers, for probation or criminal activity, for opioid avoidance testing in the clinic, or after vehicle or workplace accidents. Random drug testing can also occur at any time at a workplace, 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

What drugs are tested?

Drug tests are typically done on urine, but other types of specimens may also be used, such as hair, saliva, sweat or blood. The most commonly screened substances include:

Additional categories, as noted by SAMHSA, may include:

Consequences of false-positives

Drug screening results can negatively affect many circumstances; therefore, accuracy is of the utmost importance.

A worry for anyone undergoing an illicit drug screen - whether it be a urine, hair, saliva or blood test - is the possibility of a false positive drug test. In fact, some data suggests 5 to 10 percent of all drug tests may result in false positives and 10 to 15 percent may yield false negatives.2

The increased use of onsite, workplace random drug testing and home-testing kits emphasize the need for reliable, confirmatory testing. Negative consequences of false-positive drug testing can include1:

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

How are drug tests done?

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. This greatly lessens the chance of a false positive, almost reducing the risk to zero.3 GC-MS is a very specific test to identify separate compounds within a sample. However, GS-MS can also lead to a false negative if the GC-MS column is not designed to identify all potential compounds.1

The initial immunoassay allows 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. For instances where an initial drug screen returns a positive result, the GC-MS test is run to confirm or deny results. GC-MS is considered the “gold-standard” for drug testing

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, and vitamin, dietary supplement, and herbal products used prior to the time of the sample collection. Certified labs are use for testing to ensure quality results.

The results of your drug testing should remain confidential and kept separate from the regular employee work file.

False-positive drug test results have been reported in multiple drug classes. Some of the more common classes include cold and allergy products, pain medications, and drugs for infections:

The chart below identifies common substances reported to cause a false-positive result based on the initial immunoassay testing. Remember, 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

Substances that may interfere with drug testing*: Reported false positive result*:
amantadine (Osmolex ER) amphetamines1
amitriptyline (Elavil, brand discontinued in US) LSD (lysergic acid diethyamide)13
benzphetamine amphetamines1
brompheniramine (Dimetapp) amphetamines and methamphetamines4,8
bupropion (Aplenzin, Forfivo XL, Wellbutrin XL) amphetamines and methamphetamines8, LSD13
carbamazepine (Carbatrol, Equetro, Tegretol) tricyclic antidepressants1
chlorpromazine amphetamines and methamphetamines8, methadone8
clomipramine (Anafranil) methadone8
coca leaf tea cocaine1 - see notes below
cocaine anesthetics, topical cocaine1
cyclobenzaprine (Amrix, Fexmid) tricyclic antidepressants1
cyproheptadine tricyclic antidepressants1
dextromethorphan (Robitussin, Delsym, Nyquil, Triaminic) - see notes below phencyclidine (PCP)1,8,14 opiates1,14
diphenhydramine (Benadryl, Zzzquil), doxylamine (Unisom, Nytol) methadone8, opiates1, phencyclidine (PCP)1, tricyclic antidepressants1
diet pills amphetamines4
desipramine (Norpramin) amphetamines1
dronabinol (Marinol), nabilone (Cesamet) cannabinoids1
efavirenz (Sustiva) - also found in many combination HIV meds cannabinoids1, benzodiazepines
ephedrine nasal amphetamines and methamphetamines1,4
hemp oil, hemp food products marijuana (cannabinoids), tetrahydrocannabinol (THC)1,6
hydroxyzine (Vistaril) tricyclic antidepressants1
ibuprofen, naproxen, tolmetin (NSAIDs) - see notes below marijuana (cannabinoids), barbiturates, benzodiazepines1; Phencyclidine (PCP)1,8
imipramine (Tofranil) phencyclidine (PCP)1, LSD13
ketamine (Ketalar), possibly esketamine (Spravato), the S-enantiomer of racemic ketamine phencyclidine (PCP)1
labetalol (Trandate) amphetamines1
MDMA (illicit drug) amphetamines1
meperidine (Demerol) phencyclidine (PCP)1
mesoridazine (discontinued in US) phencyclidine (PCP)1
methylphenidate (Adhansia XR, Aptensio XR, Concerta, Daytrana, Focalin XR, Ritalin, others) amphetamines1
oxaprozin (Daypro) benzodiazepines1
pantoprazole (Protonix), possibly other proton pump inhibitors (PPIs) tetrahydrocannabinol (THC)1,5,6
phenylpropanolamine (discontinued in US) amphetamines and methamphetamines1,4,8
phentermine (Adipex-P, Lomaira, Qsymia) amphetamines1
poppy seeds - see notes below opiates and morphine1,4,8
promethazine (Phenadoz, Promethegan) amphetamines and methamphetamines1,4,8
pseudoephedrine (can be found in behind the counter nonprescription allergy and cold products such as Allegra-D, Clarinex-D, Mucinex-D) amphetamines1
quetiapine (Seroquel, Seroquel XR) methadone8, tricyclic antidepressants1
quinine opiates1
quinolone antibiotic (ofloxacin, gatifloxacin) phencyclidine (PCP)8, opiates1
ranitidine (discontinued in US) amphetamines and methamphetamines1,8
rifampin (Rifadin, Rifamate, Rimactane) opiates1,8
ritodrine (discontinued from US) amphetamines1
selegiline (Emsam, Zelapar) amphetamines1,9
sertraline (Zoloft) - see notes below benzodiazepines1,7,8
thioridazine (Thorazine - brand discontinued) methadone1,8, phencyclidine (PCP)1
tramadol (Conzip, Ultram) phencyclidine (PCP)1
trazodone (Desyrel) amphetamines and methamphetamines1,8
trimethobenzamide (Tigan, Ticon) amphetamines1
trimipramine (Surmontil) amphetamines1
venlafaxine (Effexor XR) phencyclidine (PCP)1,8
verapamil (Calan SR) methadone8, other opiates1

*Sources: References 4-9; note that Table 1 does not cover all substances that may be responsible for false positive results; above products may also be found in combination drugs; results may be assay-dependent; assays may have been reformulated at any time to prevent a false outcome.

Additional Details

Poppy Seeds

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 rolls or raw seeds in urine testing.

With the 300 ng/mL cutoff for urine testing, morphine was detected after rolls in 50 percent of samples at 20 hours, and in 100 percent of raw poppy seeds 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.

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. However, the public typically eats rolls or pastries and would have no reason or desire to consume 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 to reduce the number of opiate false positives due to poppy seed consumption. This level is current as of 2019, the most recent update as reported by Quest diagnostics.15

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 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 this possibility under the "Warnings and Precautions" section. Confirmatory tests, such as gas chromatography/mass spectrometry (GC-MS), will help to identify sertraline from benzodiazepines when assessing a suspected false positive.


Procaine or lidocaine has been noted as a defense for cocaine abuse. According to one report, benzoylecgonine is only found in nature as a metabolite of cocaine, and there would be no other valid reason for its presence in a drug screen.2

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.


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.1

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

Have Questions?

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See also


  1. Moeller K, Lee K, Kissack J. Urine Drug Screening: Practical Guide for Clinicians. Mayo Clin Proc. 2008;83:66-76. Accessed August 8, 2021 at
  2. American Psychiatric Association (APA). New Study Shows Inconsistencies In Drug Testing. Accessed August 8, 2021. 
  3. Doering, PL, Boothbay LA. Drug Testing in the Workplace: What the pharmacist should know. Drug Topics (Modern Medicine) 2003;147:63.
  4. Oliff H. Drug Tests: Don't Fall Victim to a "False-Positive". Accessed August 8, 2021 at
  5. Protonix Package Insert. May 2019. Accessed August 8, 2021 at
  6. Felton D, et al. 13-year-old girl with recurrent, episodic, persistent vomiting: out of the pot and into the fire. Pediatrics. 2015 Apr;135(4):e1060-3. Accessed August 8, 2021 at
  7. Sertraline product label (pro). Accessed August 8, 2021 at
  8. Brahm N, Yeager L, Fox M, et al. Commonly prescribed medications and potential false-positive urine drug screens. Am J Health-Syst Pharm. 2010;67:1344-50.
  9. Cody, J. D. Metabolic Precursors to Amphetamine and Methamphetamine. Forensic Science Review 1993:5(2):109–27.
  10. Quest Diagnostics. Challenging the Poppy Seed Defense. White Paper. “Concentrations of Morphine and Codeine in Paired Oral Fluid and Urine Specimens Following Ingestion of a Poppy Seed Roll and Raw Poppy Seeds. Accessed August 8, 2021 at
  11. Efavirenz product Label. Accessed August 8, 2021 at
  12. Nasky KM, Cowan GL, Knittel DR. False-positive urine screening for benzodiazepines: an association with sertraline? A two-year retrospective chart analysis. Psychiatry. 2009; 6:36-9. 
  13. Saitman A, Hyung-Doo P, Fitzgerald R. False-Positive Interferences of Common Urine Drug Screen Immunoassays: A Review. Journal of Analytical Toxicology, Volume 38, Issue 7, September 2014, Pages 387–396. Accessed June 29, 2019 at
  14. Schwebach A, Ball J. Urine Drug Screening: Minimizing False-Positives and False-Negatives to Optimize Patient Care. US Pharm. 2016;41(8):26-30. Accessed August 8, 2021 at
  15. Common Urine Drug Test Panels. Quest Diagnostics. 2/2019. Accessed August 8, 2021 at

Further information

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