Can Lamitcal Cause A False-Positive For PCP?

In our latest question and answer, the pharmacist discusses Lamictal causing false-positives for PCP.

Can Lamitcal Cause A False-Positive For PCP?
Apr 25, 2019

Madeline asked

Will Lamictal or the generic lamotrigine cause a false-positive for PCP? What other prescription drugs will cause a false-positive for PCP?

At a glance

  • Lamictal (lamotrigine) has been reported to cause false-positive test results on urine drug screenings for PCP (phencyclidine).


Drug Test Form With Text Lamictal Drug Test

Lamictal (lamotrigine) has been reported to cause false positives for PCP (phencyclidine) on urine drug screenings.

How often this could potentially occur isn't known, unfortunately, as there are only a few published case reports describing lamotrigine as the possible culprit.

Nevertheless, many studies have reported that in general, urine drug tests for PCP are notoriously unreliable and ideally, should only be used as an initial screening. If a positive result should occur, a confirmatory test (such as gas chromatography-mass spectrometry) should be done to confirm or refute the result.[1]

PCP Drug Tests

PCP (phencyclidine), a potent dissociative anesthetic, is one of the five drugs that is tested for in federally mandated work drug screening programs (the other four being amphetamines, marijuana, cocaine, and opiates).[2]

Although the potential for a false-positive result is present for any drug being tested for, most studies report that PCP has the least reliable results.

One study states the following:[3]

"Many immunoassays that are available purport to specifically identify PCP in urine specimens. However, the low prevalence of PCP use combined with the low specificity of PCP immunoassays makes the positive predictive value of PCP screen very low."

Not only has Lamictal (lamotrigine) been linked to false-positives for PCP, many other drugs have as well, including:[4]

  • Dextromethorphan
  • Benadryl (diphenhydramine)
  • Tramadol
  • Doxylamine
  • Ibuprofen
  • Effexor (venlafaxine)
  • Alprazolam
  • Coreg (carvedilol)

As mentioned above, how often a false-positive result for PCP occurs isn't known. Nevertheless, one study reported that dextromethorphan and tramadol caused erroneous results for PCP over 20% of the time.[5]

Urine Drug Screens

You may be wondering how urine drug screenings work and why they can be so unreliable.

Most urine drug screenings are 'immunoassay' tests. They rely on antibodies to react to a particular drug substance in a given sample.[6]

If a drug is present in a sample (such as PCP), an antibody will bind to it and cause a reaction that will get recorded as a positive result.

Unfortunately, antibodies can sometimes react with the wrong drug, creating a false-positive result. This is more likely to happen with drug molecules that are similar in structure, but this isn't always the case.[7]

If you do receive a result on a urine drug test that you believe is incorrect, there are far more accurate, confirmatory tests, that can be utilized. One such technique is to use gas chromatography-mass spectrometry, which can identify specific compounds in a sample, and don't rely on antibodies.

  1. ^ Lamotrigine Cross-Reactivity With Phencyclidine in Rapid Urine Toxicology in a Research Study. PubMed
  2. ^ Drug-Free Workplace Programs: Drug Testing. SAMHSA
  3. ^ False-Positive Interferences of Common Urine Drug Screen Immunoassays: A Review. Oxford Academic
  4. ^ Clinical Interpretation of Urine Drug Tests: What Clinicians Need to Know About Urine Drug Screens. Mayo Clinic Proceedings
  5. ^ How often do false-positive phencyclidine urine screens occur with use of common medications. PubMed
  7. ^ A Practical Guide to Urine Drug Monitoring. PubMed

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