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False-Positive Drug Screen Causes

By Gianna Rose ; Updated August 14, 2017

Drug screens, or tests that detect the use of controlled or illicit substances, are becoming increasingly popular at schools and in workplaces. Companies hope to identify drug use to curb increased health care costs, absenteeism, accidents and decreased productivity. Airlines and other transportation industries are required by law to administer random drug tests. Schools may test students before allowing participation in sports, and some private schools require a negative screen as a condition of admission. False-positive drug screens occur, and they can have serious consequences.

Over-the-Counter Medicines

Certain over-the-counter medicines can cause false-positive drug screen results when their structure is similar to the drug being tested for. Pseudoephedrine, a decongestant, and phenylpropanolamine, a weight loss drug, can cause a false-positive result that indicate the presence of amphetamines. Other remedies for colds, weight loss, allergies and nasal decongestants can also incorrectly indicate amphetamines. Ibuprofen, a non-steroidal anti-inflammatory drug, can cause a false-positive for marijuana, benzodiazepines such as Valium and barbiturates, such as phenobarbitol.


Certain foods can result in false-positive drug screen results. Narcotics are made from poppies, and consuming poppy seeds can cause a false-positive result indicating presence of the drugs, although a large amount of the seeds has to be consumed. A poppy seed bagel does not contain enough of the seeds to skew test results, according to New York University's Langone Medical Center. Foods made with hemp and hemp oil, taken for its nutritional benefits, can cause a drug screen to incorrectly indicate marijuana.

Lab Errors

Lab errors are the cause of some false-positive drug screens. During collection of a specimen, contamination of the container or use of alcohol to clean the skin can affect the results. Other causes of false-positive results include incorrectly identifying the test subject, mislabeling the specimen, specimen mix-ups and technician error in interpreting the results, the Substance Abuse and Mental Health Services Administration explains. These same factors can also cause inconclusive and false-negative results as well.

Prescription Medications

Drug screen results can be affected by certain prescription medications. Amoxicillin, an antibiotic, can cause a false-positive result for cocaine, according to Net Wellness. An article titled "The False-Positive Urine Screening for Benzodiazepines: An Association with Sertraline?" in the July 2009 issue of the journal "Psychiatry" by Kevin M. Nasky, D.O., and colleagues reports study findings that suggest sertraline, the brand-name antidepressant Zoloft, can cause a false-positive result for benzodiazepines with a commonly used screening test. Effexor, an antidepressant known generically as venlafaxine, produced a false result indicating the hallucinogen phencyclidine, or PCP, says a letter to the editor from Patricio Molero Santos, M.D., and colleagues in the February 2007 issue of The American Journal of Psychiatry.

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