Can Cetirizine, Coamoxiclav, or Isoniazid Cause False-Positive Drug Tests for Methamphetamine or Marijuana?
None of these three medications are documented to cause false-positive results for methamphetamine or marijuana on standard urine drug screens.
Evidence for Each Medication
Cetirizine (Antihistamine)
- Cetirizine is not identified in the medical literature as causing false-positive drug screen results 1
- While other antihistamines like diphenhydramine, brompheniramine, and doxylamine have been reported to cause false-positive amphetamine results, cetirizine is notably absent from these reports 2
- The guideline literature on urticaria management discusses cetirizine extensively but makes no mention of drug test interference 3
Coamoxiclav (Amoxicillin/Clavulanic Acid)
- Amoxicillin with clavulanic acid is not reported to interfere with methamphetamine or marijuana drug screens 4, 2
- While this antibiotic combination can cause hepatotoxicity, there is no documented cross-reactivity with amphetamine or cannabinoid immunoassays 5
- Fluoroquinolone antibiotics (not amoxicillin) are the antibiotic class known to cross-react with opiate screens, but this does not affect amphetamine or marijuana testing 3, 6
Isoniazid (Antituberculous Drug)
- Isoniazid is mentioned in anxiety disorder guidelines as a monoamine oxidase inhibitor (MAOI) that can contribute to serotonin syndrome, but not as a cause of false-positive drug screens 3
- The tuberculosis treatment guidelines do not mention any interference with drug testing 3
- Comprehensive reviews of medications causing false-positive drug screens do not list isoniazid 2
Medications That DO Cause False-Positive Amphetamine Results
For clinical context, the following medications are documented to cause false-positive amphetamine/methamphetamine results:
- Pseudoephedrine in over-the-counter cold medications 3, 7
- Bupropion (antidepressant) 7
- Ranitidine (H2-blocker) 2
- Trazodone (antidepressant) 2
- Promethazine (antihistamine) 2
Critical Clinical Recommendations
When Interpreting Unexpected Results
- Always obtain a complete medication history before interpreting drug test results, as many medications can cause false positives 1
- Request confirmatory testing using gas chromatography-mass spectrometry (GC-MS) when results are unexpected or will impact patient management, as this method can definitively distinguish between true drug use and false positives 1, 6, 7
- Discuss unexpected results with laboratory personnel or toxicologists before making clinical decisions that could have serious consequences 1, 6
Important Caveats
- Standard immunoassay drug tests are susceptible to cross-reactions that limit their specificity 7
- Many healthcare providers have inadequate training in interpreting urine drug test results, and incorrect interpretation can have severe consequences including loss of child custody or legal implications 1, 6
- Never dismiss patients or make punitive decisions based solely on a single drug test result without confirmatory testing 3, 1
- Establishing a working relationship with your local toxicologist is recommended for consultation on complex results 1, 6
Bottom Line
If your patient taking cetirizine, coamoxiclav, or isoniazid has a positive drug screen for methamphetamine or marijuana, these medications are not the cause. You should proceed with confirmatory GC-MS testing and consider other explanations, including actual drug use, other medications not disclosed, or laboratory error 1, 6, 2.