What over-the-counter (OTC) medications, such as Benadryl (diphenhydramine), can cause a false positive result in an opiate drug test?

Over-the-Counter Medications That Can Cause False Positive Opiate Drug Tests

Several over-the-counter medications can cause false positive results on opiate drug tests, with quinolone antibiotics and certain antihistamines being the most common culprits. 1

Common OTC Medications Causing False Positive Opiate Results

Antihistamines

• Diphenhydramine (Benadryl) - Can cause false positives on standard immunoassay-based opiate tests 1
• The American Academy of Pediatrics notes that certain antihistamines can cross-react with standard drug screens, leading to false positive results 1

Decongestants and Cough Medications

• Dextromethorphan - Found in many OTC cough medications, can potentially cause false positive results 1
• Pseudoephedrine - Common decongestant that may trigger false positives 1

Other Notable OTC Products

• Poppy seeds - Consumption can lead to false positive opiate results due to trace amounts of morphine and codeine 1
• Some herbal supplements - Certain herbal products may interfere with drug testing methodology 1

Mechanism of False Positives

False positives typically occur through one of these mechanisms:

1. Cross-reactivity - The chemical structure of the medication resembles opiates enough to trigger the antibodies in immunoassay tests 1
2. Metabolic interference - Some medications affect the metabolism of compounds tested in drug screens 1
3. Laboratory error - Improper handling or testing procedures 1

Detection Methods and Limitations

Standard urine drug screens typically test for substances including amphetamines, barbiturates, benzodiazepines, cocaine, marijuana, methadone, opiates, and others, but have limitations:

• Immunoassay-based tests are relatively inexpensive and fast but are susceptible to cross-reactions causing false positives 1
• More accurate confirmation testing requires specialized methods like gas chromatography/mass spectrometry (GC-MS) or liquid chromatography/mass spectrometry (LC-MS/MS) 1

Research Evidence on Specific Medications

Quinolone Antibiotics

• While not strictly OTC in the US, quinolones like ofloxacin have been documented to cause false positive opiate results at concentrations as low as 200 mg/L (0.16% cross-reactivity) 2
• A comprehensive study found that 9 out of 13 tested quinolones caused assay results above the threshold for a positive opiate result in at least one common testing system 3

Dextromethorphan

• Contrary to some beliefs, research has shown that a single normal dose (or even twice normal dose) of dextromethorphan is unlikely to produce a false positive opiate result on EMIT screens 4

Clinical Implications

When interpreting drug test results:

• Always document any medications the patient is currently taking to help interpret results 1
• Unexpected positive results should prompt a conversation with the patient rather than immediate punitive action 1
• Consider confirmatory testing with GC-MS or LC-MS/MS for definitive results when false positives are suspected 1

Avoiding Misinterpretation

To prevent misinterpretation of drug test results:

• Collect detailed medication history, including all OTC products
• Use confirmatory testing when results are unexpected or disputed
• Be aware that standard "opiate" panels have limitations and may produce false positives with certain medications 1
• Consider the detection windows for various substances when interpreting results 1