Can Benadryl Cause False Positives on Urine Drug Screens?
Yes, diphenhydramine (Benadryl) can cause false-positive results on urine drug screens, most notably for methadone, phencyclidine (PCP), and tricyclic antidepressants (TCAs).
Understanding the Mechanism
Diphenhydramine causes false positives through cross-reactivity with immunoassay screening tests 1. These rapid screening tests use competitive binding principles that can mistake diphenhydramine's chemical structure for other substances 2. The guideline literature emphasizes that false-positive results are more likely to occur on initial screening tests because of cross-reactivity with unrelated substances in the urine 3.
Specific False-Positive Patterns
Methadone
- Most clinically documented: Diphenhydramine has demonstrated clear cross-reactivity with methadone immunoassays 2
- Laboratory testing confirmed that diphenhydramine itself (not its metabolites nordiphenhydramine or dinordiphenhydramine) causes the false-positive methadone result 2
- This has been documented in pediatric emergency department cases where patients with diphenhydramine ingestion tested positive for methadone despite no exposure history 2
Phencyclidine (PCP)
- Diphenhydramine is associated with false-positive PCP screens, though the association was not statistically significant in one retrospective study 4
- Tramadol and dextromethorphan showed stronger associations, but diphenhydramine remains a recognized interferent 4
Tricyclic Antidepressants (TCAs)
- Recent evidence (2025) demonstrates that diphenhydramine metabolites—specifically diphenhydramine N-oxide (DPH-NO) and diphenhydramine N-glucuronide (DPHG)—cause false-positive TCA results 5
- In overdose cases, urinary concentrations of DPH-NO (110-170 µg/mL) and DPHG (28-370 µg/mL) were associated with false-positive TCA results 5
Amphetamines
- Diphenhydramine is listed among antihistamines that have been reported to cause false-positive amphetamine/methamphetamine results 1
Clinical Implications and Management
Always obtain confirmatory testing when a positive screening result doesn't match the clinical picture 3. The guidelines are explicit: screening immunoassays are presumptive only, and confirmatory tests using gas chromatography-mass spectrometry (GC-MS) are highly specific and will reliably distinguish true positives from false positives 3, 1, 6.
Key Practice Points:
- Document medication history: Know the patient's complete medication list, including over-the-counter medications like diphenhydramine 3
- Interpret with caution: When screening results are positive for methadone, PCP, or TCAs in patients taking diphenhydramine, suspect cross-reactivity 2, 4, 5
- Confirm before acting: Do not make clinical decisions based solely on immunoassay screening results when false positives are possible 3, 6
- Contact the laboratory: Seek assistance from the testing laboratory when results don't correlate with clinical findings 3
Common Pitfalls to Avoid
Acting on screening results alone: Immunoassay screens are designed for high sensitivity, not specificity—false positives are expected and require confirmation 6
Overlooking OTC medications: Diphenhydramine is available in over 300 formulations, most over-the-counter 7, making it easy to miss in medication histories
Assuming all metabolites behave the same: Only diphenhydramine itself causes methadone false positives, while its metabolites (DPH-NO and DPHG) specifically cause TCA false positives 2, 5
Ignoring timing and concentration: False positives are more likely with higher diphenhydramine concentrations, particularly in overdose scenarios 5
The bottom line: Confirmatory testing with GC-MS is mandatory when positive screening results conflict with clinical assessment or medication history, as immunoassays cannot reliably distinguish diphenhydramine from methadone, PCP, or TCAs 3, 1, 6.