Can a Drug Test Lead to a False Positive?
Medically reviewed by L. Anderson, PharmD Last updated on Jun 27, 2019.
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. 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
Drug tests are typically done on urine, but other types of specimens are also used, such as hair, saliva or blood. The most commonly screened substances, and also required by Federal workplace guidelines, include these five categories of drugs:
Additional categories, as noted by SAMHSA, may include:
- Designer drugs like Bath Salts, Spice, or Ecstasy (MDMA)
- Benzodiazepines, like diazepam (Valium) or alprazolam (Xanax)
- Ethanol (alcohol)
- Prescription opioids like hydrocodone, oxycodone, methadone
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, previous 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.
When initial screening drugs tests (called immunoassays) result in positive results, a second confirmatory (Gas Chromatography Mass Spectrometry or GC-MS) test - which should always be done on positive results - 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 on this additional saved portion 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. 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:
- 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. 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
|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|
|brompheniramine (Dimetapp)||amphetamines and methamphetamines4,8|
|bupropion (Aplenzin, Forfivo XL, Wellbutrin XL, Zyban)||amphetamines and methamphetamines8, LSD13|
|carbamazepine (Carbatrol, Equetro, Tegretol)||tricyclic antidepressants1|
|chlorpromazine||amphetamines and methamphetamines8, methadone8|
|coca leaf tea||cocaine1|
|cocaine anesthetics, topical||cocaine1|
|cyclobenzaprine (Amrix, Fexmid, Flexeril [brand discontinued in US])||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|
|dronabinol (Marinol), nabilone (Cesamet)||cannabinoids1|
|efavirenz (Sustiva) - also found in many combination HIV meds||cannabinoids1, benzodiazepines|
|ephedrine nasal||amphetamines and methamphetamines1,4|
|hemp oil||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|
|MDMA (illicit drug)||amphetamines1|
|meperidine (Demerol)||phencyclidine (PCP)1|
|mesoridazine (brand discontinued in US)||phencyclidine (PCP)1|
|methylphenidate (Adhansia XR, Aprensio XR, Concerta, Daytrana, Focalin XR, Ritalin, others)||amphetamines1|
|pantoprazole (Protonix), possibly other proton pump inhibitors (PPIs)||tetrahydrocannabinol (THC)1,5,6|
|phenylpropanolamine (brand 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 (Phenergan [brand discontinued in US], Phenadoz, Promethegan)||amphetamines and methamphetamines1,4,8|
|pseudoephedrine (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|
|quinolone antibiotic (ofloxacin, gatifloxacin)||phencyclidine (PCP)8, opiates1|
|ranitidine (Zantac 75, Zantac 150)||amphetamines and methamphetamines1,8|
|rifampin (Rifadin, Rifamate, Rimactane)||opiates1,8|
|selegiline (Eldepryl [brand discontinued in US], Emsam, Zelapar)||amphetamines1,9|
|sertraline (Zoloft) - see notes below||benzodiazepines1,7,8|
|thioridazine (Thorazine)||methadone1,8, phencyclidine (PCP)1|
|tramadol (Conzip, Ultram)||phencyclidine (PCP)1|
|trazodone (Desyrel)||amphetamines and methamphetamines1,8|
|trimethobenzamide (Tigan, Ticon)||amphetamines1|
|venlafaxine (Effexor XR)||phencyclidine (PCP)1,8|
|verapamil (Calan)||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 to prevent chance of false-positive outcome.
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 2015 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, both 300 and 1,200 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 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
Oral and urine drug testing can typically detect opiates up to 48 to 72 hours after use.
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.
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 and possibly opiates. 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 does not bind to cannabinoid receptors. However, as noted in the efavirenz (Sustiva) package insert, false-positive tests for cannabinoids (THC) have been reported when the CEDIA DAU Multilevel THC assay is used. False-positive results with other cannabinoid assays have not been observed. False-positive tests for benzodiazepines have also been reported and are likely due to the 8-hydroxy-efavirenz major metabolite.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 are easily available OTC 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.
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 GC-MS. They found 26 of 98 records to be identified as false positives. The package insert for sertraline now warns about this possibility under the ‘Laboratory Tests’ section. Confirmatory tests, such as gas chromatography/mass spectrometry, will help to identify sertraline from benzodiazepines when assessing a suspected false positive.
Novocaine or Lidocaine
Procaine (brane name: Novocain) or lidocaine use as a defense for cocaine abuse is not a valid excuse. If benzoylecgonine, the main metabolite of cocaine is detected in a drug test, the subject cannot claim that the result is a false positive due to Novocain administration, or any other "-caine" drug. 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 the cut-off concentrations for lab analysis (usually 50 ng/mL) are set well above that which might occur from passive inhalation.
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 There is the possibility pH may be altered, as well.
Testing for adulterants in addition to drugs may be requested by the person requiring the drug screen.
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:
- Anabolic Steroids - Abuse, Side Effects and Safety
- Blood Doping: Lance Armstrong & Pro Cycling
- Drug Testing FAQs
- Toxicology Drug Testing
- Moeller K, Lee K, Kissack J. Urine Drug Screening: Practical Guide for Clinicians. Mayo Clin Proc. 2008;83:66-76. Accessed June 29, 2019 at https://www.mayoclinicproceedings.org/article/S0025-6196(11)61120-8/fulltext
- American Psychiatric Association (APA). New Study Shows Inconsistencies In Drug Testing. https://www.rttnews.com/amp/1323040/new-study-shows-inconsistencies-in-drug-testing.aspx Accessed June 29, 2019.
- Doering, PL, Boothbay LA. Drug Testing in the Workplace: What the pharmacist should know. Drug Topics (Modern Medicine) 2003;147:63.
- Oliff H. Drug Tests: Don't Fall Victim to a "False-Positive". Accessed May 21, 2016 at http://health.cvs.com/GetContent.aspx?token=f75979d3-9c7c-4b16-af56-3e122a3f19e3&chunkiid=13837
- Protonix Package Insert. May 2019. Accessed June 29, 2019 at https://www.drugs.com/pro/protonix.html
- 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 June 29, 2019 at https://www.ncbi.nlm.nih.gov/pubmed/25733759
- Sertraline product label (pro). Drugs.com. Accessed June 29, 2019 at https://www.drugs.com/pro/sertraline.html
- 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.
- Cody, J. D. Metabolic Precursors to Amphetamine and Methamphetamine. Forensic Science Review 1993:5(2):109–27.
- 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 June 29, 2019 at https://blog.employersolutions.com/challenging-the-poppy-seed-defense/
- Efavirenz product Label. Drugs.com. Accessed June 29, 2019 at https://www.drugs.com/ppa/efavirenz.html
- 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. Accessed June 29, 2019.
- 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 https://academic.oup.com/jat/article/38/7/387/2798054
- 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 June 29, 2019 at https://www.uspharmacist.com/article/urine-drug-screening-minimizing-false-positives-and-false-negatives-to-optimize-patient-care
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