Do Klonopin Or Benadryl Cause A False-Positive PCP Test?
In our latest question and answer, the pharmacist whether Benadryl and Klonopin have been reported to cause false positives for PCP on urine drug tests.
Why do I test positive for PCP and do not use that drug at all?! I have been taking Klonopin and Benadryl. My doctor keep telling me somebody around me must use it but nothing of what she say is true. I do not in the name of Jesus use PCP. What can you tell me to tell my doctor in order for her to study this false positive urine test readings for PCP?
Jun 21, 2021- Both Klonopin and Benadryl are known to cause false-positive results for PCP on urine drug screens.
- If you get a positive PCP result, tell your doctor and the testing lab about all medications you're taking, including over-the-counter drugs and supplements, as this information can help them investigate.
- More specific tests like gas chromatography or mass spectrometry can distinguish between a false positive and a true positive result.
Hi Raerae and thank you for your question.
Even though you didn’t mention the type of test, urine drug screens are the most common, easiest, and cheapest way to screen for drugs, so I am going to assume that’s the case here. Urine drug screenings typically aren’t specifically looking for a specific drug in your urine. They are usually looking for a known metabolite, or a known end product that your body would turn a drug into before you urinate it out.
Due to the fact that a urine drug screen is usually looking for a metabolite and probably not the actual drug itself, similar metabolites, sometimes from unrelated drugs may cause a false positive. Most labs may have a reference file of drugs that may cause a false positive. If I were in your shoes, I would provide the lab or your physician with a list of medications that you are presently taking, as well as any over the counter herbals or supplements.
There are a few commonly used over the counter drugs that seem to produce a false positive for phencyclidine (PCP, angel dust, etc.). You specifically mentioned that you take diphenhydramine (Benadryl). Diphenhydramine is known to cause a false positive for PCP, so that may be the culprit in this situation.
In fact, more than one published study reports that both Klonopin (clonazepam) and Benadryl (diphenhydramine) can potentially cause false-positive results for PCP on urine screenings. Here is an excerpt from the Journal 'Clinical Toxicology':
False positive urine screens for PCP are common with tramadol, dextromethorphan, alprazolam, clonazepam, and carvedilol and may also occur with diphenhydramine.
In terms of confirming the results of a urine drug screen, more accurate tests are needed. The lab may be able to perform a gas chromatography or mass spectrometry (these are more specific tests) which may be able to help distinguish between a false positive and true positive test as these tests actually identify specific compounds in samples.
Other Drugs Causing PCP False-Positives
For reference, the Mayo Clinic, in their comprehensive and widely referenced study 'Urine Drug Screening: Practical Guide for Clinicians' lists all of the following drugs that have been reported to cause false-positives for PCP:
- Dextromethorphan
- Diphenhydramine
- Doxylamine
- Ibuprofen
- Imipramine
- Ketamine
- Meperidine
- Mesoridazine
- Thioridazine
- Tramadol
- Venlafaxine
- O-desmethylvenlafaxine
I hope this information helps and feel free to write to us in the future! Good luck!
References & sources
- False-Positive Interferences of Common Urine Drug Screen Immunoassays: A Review. — Journal of Analytical Toxicology
- Urine Drug Screening: Practical Guide for Clinicians — Mayo Clinic Proceedings
- False Positive Urine Screens for Phencyclidine. — Maryland Poison Center
Glossary — Terms in This Answer
- Metabolite
- A substance produced when your body breaks down and processes a drug.
- False positive
- A test result that shows the presence of a drug when the person hasn't actually taken it.
- Diphenhydramine
- An over-the-counter antihistamine commonly sold under the brand name Benadryl, used to treat allergies and sleep issues.
- Clonazepam
- A prescription medication sold under the brand name Klonopin, used to treat anxiety and seizures.
- Gas chromatography
- A laboratory testing method that separates and identifies specific chemical compounds in a sample with high accuracy.
- Mass spectrometry
- A laboratory testing technique that determines the exact molecular structure and identity of compounds in a sample.
- Phencyclidine
- A dangerous illegal drug also known as PCP or angel dust.
Dr. Brian Staiger, PharmD
Doctor of Pharmacy, board certified clinical pharmacist (BCPS), American Herbalist Guild, Medication Therapy Management certified
Brian Staiger, PharmD, BCPS, is a licensed pharmacist with more than 13 years of experience across retail pharmacy, clinical pharmacy, medication safety, pharmacy program development, and healthcare administration. He is the founder of HelloPharmacist.com, where he helps patients...
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