Lamotrigine, sold under the brand name Lamictal, is an anticonvulsant also prescribed as a mood stabilizer for bipolar disorder, as described in the FDA labeling for lamotrigine tablets on DailyMed. It is not one of the substances that a standard 5, 10, or 12 panel workplace immunoassay is designed to detect. Lamotrigine is not an opioid, a benzodiazepine, an amphetamine, or a cannabinoid, so a screening cup or dip card is not looking for it. That is the short answer to whether Lamictal itself will register as a drug test result.
The complication is a different one. A number of published case reports describe lamotrigine cross reacting with the antibody used in some rapid phencyclidine (PCP) immunoassay strips, producing a non-negative or presumptive positive PCP screen in patients who had not used PCP. This page explains what that cross reactivity is, why it happens on the screening step and not on laboratory confirmation, and what an employer or clinician should do when a donor taking lamotrigine screens non-negative for PCP.
What the published case reports actually found
The lamotrigine and PCP cross reactivity is not a theory, it is documented in the emergency medicine and toxicology literature. One case report published in the International Journal of Emergency Medicine describes a patient on lamotrigine whose rapid urine toxicology screen read presumptive positive for phencyclidine, a result that did not hold up once the specimen went to confirmation testing. Similar reports have been published in emergency medicine journals describing the same pattern with different immunoassay platforms. The common thread across these reports is always the same: the screening immunoassay flagged PCP, and a second, more specific method cleared it.
Lamotrigine is not the only medication reported to cross react with the PCP assay. Venlafaxine, the active ingredient in Effexor, has also been linked to PCP false positives on some screening platforms, which is a useful comparison for understanding how these antibody based tests can be fooled by molecules that only loosely resemble the target drug. If you want the fuller picture on how venlafaxine behaves on a screen, see our related post on Effexor and PCP false positives.
Screening immunoassay versus confirmation testing
Every non-negative drug test result should be understood as coming from one of two very different steps, and the difference matters for exactly this kind of situation.
The screening step
A drug test cup, dip card, or oral fluid device uses an immunoassay. Antibodies in the test are built to bind to a target drug or its metabolites, and if enough of that substance (or something that resembles it closely enough at the molecular level) is present above a set cutoff concentration, the line does not appear and the result reads non-negative. Immunoassay screening is fast, inexpensive, and built for high volume use, but it is a presumptive method. It can be fooled by structurally similar compounds, which is exactly the mechanism behind the lamotrigine and PCP reports.
The confirmation step
Any non-negative screen that matters for an employment or safety decision should go to a laboratory for confirmation using gas chromatography mass spectrometry (GC-MS) or liquid chromatography tandem mass spectrometry (LC-MS/MS). These methods identify the exact molecule present, not just a family of molecules that bind similarly to an antibody. Confirmation testing is what separates a true PCP positive from a lamotrigine driven false alarm. A donor taking lamotrigine as prescribed will not have phencyclidine or its metabolites in a confirmation test, because there is no PCP in their system to find.
| Factor | Screening immunoassay | Laboratory confirmation (GC-MS or LC-MS/MS) |
|---|---|---|
| Purpose | Rapid, presumptive first pass | Definitive identification of the specific substance |
| Where it happens | Point of collection cup, dip card, or oral fluid device | Certified toxicology laboratory |
| Result type | Non-negative or negative against a cutoff | Positive, negative, or specimen invalid, tied to an exact analyte |
| Vulnerable to cross reactivity | Yes, structurally similar drugs and some medications can trigger a line | No, the method identifies the molecule itself |
| Who reviews a non-negative result before it becomes final | Not final on its own | Medical review officer (MRO) reviews the confirmed result and any prescription documentation |
Why the confirmed positive rate for PCP is so low even though panels still screen for it
PCP remains one of the five analytes in the federal drug testing panel described in the Substance Abuse and Mental Health Services Administration's drug free workplace program, and it stays on most commercial 5, 10, and 12 panel cups for the same reason. Actual PCP use is uncommon compared to other drug classes, but the analyte has a long history in regulated testing and cross reactivity issues like the lamotrigine reports are exactly why the confirmation step exists as a mandatory part of the process rather than an optional add on.
What the medical review officer does with a non-negative PCP screen
In any drug testing program that follows accepted practice, a non-negative screening result is never the end of the process. The specimen goes to confirmation, and if the laboratory reports it as positive, a medical review officer contacts the donor before the result is reported to the employer. The MRO asks about current prescriptions, reviews documentation from the prescribing provider or pharmacy, and determines whether a legitimate medical explanation accounts for the result. This is the process laid out for federally regulated transportation employers in 49 CFR Part 40, and it mirrors the review structure that the Mandatory Guidelines for Federal Workplace Drug Testing Programs established for federal agencies. Because lamotrigine cross reactivity happens at the screening step, the far more likely outcome for a patient taking Lamictal as prescribed is that the laboratory confirmation itself comes back negative for PCP, and the MRO never needs to reach a prescription question at all. If a donor is asked about a non-negative screen, the prescription label and pharmacy record for lamotrigine are exactly what an MRO would want to see to resolve the question quickly. A full walkthrough of how that review works is in our post on the medical review officer process.
Employers who are not subject to DOT rules generally are not required to use an MRO, but the same logic still applies. A single screening cup result is not proof of drug use. It is a flag that should route to confirmation testing before any employment decision is made, and a written policy that spells out how non-negative screens are handled protects both the employer and the employee.
What an employer should do when a screen flags PCP on a donor taking lamotrigine
- Do not treat a point of collection screening result as final. Send the specimen, or the split specimen already collected with the cup, to a certified laboratory for GC-MS or LC-MS/MS confirmation.
- If a confirmed positive result comes back and the donor discloses a lamotrigine prescription, route the result through a medical review officer or a comparable clinical reviewer so prescription documentation can be checked against the confirmed analyte.
- Apply the same review process consistently to every donor. Do not make exceptions based on job title or tenure.
- Keep in mind that drug testing rules and required procedures vary by state and by industry. Employers should confirm their obligations with counsel or their state labor agency rather than assuming a single national standard applies.
What a patient taking lamotrigine should do before a test
Bring the prescription bottle or a pharmacy printout to the collection site, or have it available if a reviewer contacts you afterward. This does not change how the screening cup performs, since the cross reactivity happens at the antibody level regardless of documentation, but it gives the confirmation and review process what it needs to resolve a false alarm quickly. This is general information only. It is not medical advice, and no one should stop or change a prescribed medication because of how it might affect a drug test.
Frequently asked questions
Does Lamictal itself show up as a positive on a standard drug test panel?
No. Lamotrigine is not a targeted analyte on 5, 10, or 12 panel immunoassays. It is not screened for the way opioids, amphetamines, benzodiazepines, or THC are.
Why would lamotrigine cause a PCP false positive?
Published case reports describe lamotrigine cross reacting with the antibody used in certain rapid PCP immunoassay strips, causing the screening line to read non-negative even though no phencyclidine is present. The exact rate varies by assay manufacturer and is not something a package insert will quantify precisely.
Will a confirmation test also show PCP if I take lamotrigine?
Confirmation testing with GC-MS or LC-MS/MS identifies the specific molecule present. Since lamotrigine is not phencyclidine, a properly run confirmation test on a donor who has not used PCP should not confirm a PCP positive from lamotrigine alone.
Do I need to tell the lab or my employer that I take Lamictal before I test?
You are not generally required to disclose prescriptions before testing, but having documentation available speeds up review if a screen comes back non-negative. A medical review officer or clinical reviewer will typically ask about current medications only after a confirmed positive result.
Can an employer take action based only on the initial cup result?
Employers should treat a screening result as presumptive, not final, and send any non-negative result for laboratory confirmation before making an employment decision. Practices vary by employer policy and by state law, so specific obligations should be confirmed with counsel.
Which product should I use if I want to reduce presumptive PCP results tied to cross reactivity?
No screening immunoassay eliminates cross reactivity entirely, since it is inherent to how antibody based tests work. What matters is pairing screening with confirmation and MRO or clinical review for any result that will be used to make a decision. Our drug test cup collection includes multi panel cups built for that two step workflow.
This article is general information about drug testing science and workplace procedure. It is not medical advice or legal advice. Anyone with questions about a specific medication, prescription, or test result should talk to their prescribing clinician, a medical review officer, or an attorney familiar with their state's laws.
