Medications Causing False-Positive Methadone and Benzodiazepine Urine Drug Screens
Several commonly prescribed medications can cause false-positive results for both methadone and benzodiazepines on immunoassay-based urine drug screens, with diphenhydramine, doxylamine, vortioxetine, and sertraline being the most clinically relevant culprits.
False-Positive Methadone Results
The following medications have been documented to cross-react with methadone immunoassays:
Antihistamines (Most Common)
Diphenhydramine - This over-the-counter antihistamine shows direct cross-reactivity with methadone screening assays 1, 2. Laboratory testing confirmed that diphenhydramine itself (not its metabolites nordiphenhydramine or dinordiphenhydramine) triggers false positives on immunoassay platforms 2.
Doxylamine - This common sleep aid can cause false-positive methadone results, particularly at toxic levels 3. This is clinically significant as doxylamine is prone to abuse and overdose.
Antidepressants
- Vortioxetine (Trintellix®/Brintellix®) - This is particularly important because routine therapeutic doses can cause false positives even without detectable parent drug in urine, due to high cross-reactivity of its metabolites with the Roche KIMS Methadone II assay 4. Given that vortioxetine is commonly prescribed for mood disorders—which have high prevalence in patients treated for opioid addiction—this interference has significant clinical implications 4.
Other Medications
Additional medications reported to cause false-positive methadone results include verapamil, quetiapine, and diphenhydramine-containing products 1.
False-Positive Benzodiazepine Results
Sertraline is the most well-documented medication causing false-positive benzodiazepine screens 5. In a two-year retrospective analysis, 26 out of 98 false-positive benzodiazepine results (that were not explained by concomitant benzodiazepine prescriptions) were associated with sertraline use 5.
Critical Clinical Considerations
Immunoassay Limitations
All false-positive results occur with immunoassay screening tests due to cross-reactivity; confirmatory testing with gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-tandem mass spectrometry (LC-MS/MS) will not confirm these false positives 6, 1, 2, 5. The guidelines emphasize that immunoassays are prone to false positives because they rely on competitive binding principles 6, 1.
Essential Practice Points
- Always obtain a complete medication history (including over-the-counter medications and supplements) before interpreting urine drug screen results 6
- Confirmatory testing is mandatory when results don't correlate with clinical findings or when consequences are significant 6, 1
- Consult the testing laboratory to understand which specific substances are included in your panel and potential cross-reactivities 6
Common Pitfall
A "clinical false-positive" can also occur when patients legitimately take prescribed medications that metabolize into substances on the testing panel (e.g., amphetamine/dextroamphetamine for ADHD testing positive for amphetamines) 6. This differs from true cross-reactivity but requires the same careful interpretation—drug testing cannot distinguish between appropriate use and misuse of prescribed medications 6.
Designer Benzodiazepines
Be aware that non-FDA-approved designer benzodiazepines (flubromazolam, flualprazolam, etizolam, etc.) can trigger positive benzodiazepine screens but may not be detected by standard LC-MS/MS confirmatory panels, potentially leading to misinterpretation of screening results 7.