What Can Cause a False Positive for PCP?

Several common prescription and over-the-counter medications can trigger a false-positive result for phencyclidine (PCP) on a standard urine drug screen. The most frequently documented culprits include the antidepressant venlafaxine and its derivative desvenlafaxine, the cough suppressant dextromethorphan, the pain reliever tramadol, the antihistamine diphenhydramine, and the seizure medication lamotrigine. These false positives happen because standard screening tests use antibodies that react to molecular shapes rather than to specific chemicals, and the shapes of these medications overlap enough with PCP to fool the test.

Why Screening Tests Get Tricked

The urine drug screens used in most hospitals, workplaces, and treatment programs are immunoassays. They work by using antibodies designed to latch onto PCP molecules. The problem is that antibodies are not perfectly selective. If another substance has a similar enough three-dimensional structure, the antibody grabs onto it and generates a positive signal. This is called cross-reactivity, and it is the root cause of virtually every PCP false positive. Different test manufacturers use different antibody formulations, which means a medication that triggers a false positive on one brand of test kit may not trigger it on another. That variability makes it hard to publish a definitive, universal list of interfering substances, because the answer partly depends on which test kit the lab uses.

Venlafaxine and Desvenlafaxine

The antidepressant venlafaxine, sold under the brand name Effexor, is one of the best-documented causes of false-positive PCP results. Its major metabolite, called O-desmethylvenlafaxine, appears to cross-react with PCP immunoassay reagents. Laboratory experiments have shown that preparing concentrations of either venlafaxine or this metabolite in urine produces positive PCP results, confirming the cross-reactivity is chemical rather than incidental.¹

Desvenlafaxine, marketed as Pristiq or Khedezla, deserves separate mention. This drug is itself the active metabolite of venlafaxine, packaged as a standalone antidepressant. Because it is chemically the same compound that causes venlafaxine’s cross-reactivity, it can produce the same false-positive result on its own. Case reports have documented this happening with standard screening kits.² If you take either of these medications and face a urine drug test, it is worth mentioning your prescription to the testing facility beforehand.

Dextromethorphan

Dextromethorphan, commonly abbreviated DXM, is the cough suppressant found in dozens of over-the-counter cold and flu products. It is one of the most commonly reported causes of false-positive PCP screens.³ In a study of urine samples that initially screened positive for PCP but failed confirmatory testing, about one in four of those patients had documented dextromethorphan use.⁴ That figure makes DXM one of the single largest contributors to false-positive PCP results in clinical settings.

This is worth knowing because dextromethorphan is in a huge range of products. If you picked up a bottle of Robitussin DM, NyQuil, Delsym, or any generic cough syrup with “DM” on the label before a drug test, it could be enough to trigger a positive PCP screen. The cross-reactivity makes pharmacological sense: dextromethorphan acts on some of the same brain receptors as PCP, and the two molecules share structural features that immunoassay antibodies can confuse.

Tramadol

Tramadol is a prescription pain reliever often considered a “milder” opioid, but on a PCP screen it can look like something else entirely. Case reports have demonstrated that both tramadol and its active metabolite, N-desmethyltramadol, cross-react with PCP immunoassays, and this has been confirmed through in vitro testing.⁵ In the same study of unconfirmed PCP-positive screens mentioned above, roughly one in five of those false-positive patients had documented tramadol use.⁴

Tramadol is a particularly tricky situation because it is sometimes prescribed in pain management programs that also require routine urine drug testing. A patient following their prescription as directed could end up with a result that looks like illicit PCP use, which underscores why confirmatory testing matters whenever a screen comes back positive.

Diphenhydramine

Diphenhydramine is the active ingredient in Benadryl, as well as many over-the-counter sleep aids like ZzzQuil and Unisom SleepGels. It showed up frequently among patients whose PCP screens failed confirmatory testing. In that same clinical dataset, about 15% of patients with unconfirmed PCP-positive results had documented diphenhydramine use.⁴ Because diphenhydramine is so widely available and used casually for allergies or insomnia, many people would never think to mention it when facing a drug test.

Lamotrigine

Lamotrigine, sold under the brand name Lamictal, is an anticonvulsant prescribed for epilepsy and as a mood stabilizer for bipolar disorder. It has been documented as a cause of false-positive PCP results on at least one specific immunoassay platform, the Bio-Rad TOX/See screen.⁶ This is a medication that millions of people take daily for serious neurological and psychiatric conditions, and a false PCP result can create real problems in clinical decision-making, especially in emergency departments where rapid toxicology screens guide treatment.

The lamotrigine finding also illustrates the earlier point about assay variability. The documented false positive occurred on a specific brand of test. Someone taking lamotrigine who is screened with a different manufacturer’s kit may not have the same problem. But since you rarely know which kit a given facility uses, it is still a risk worth being aware of.

Antipsychotic Medications

The older antipsychotic thioridazine has been reported to cause false-positive PCP results. One case report found that the combined presence of thioridazine and its metabolite mesoridazine triggered a positive PCP screen on the EMIT II assay, a result that was not confirmed when the sample was retested with a more specific method.⁷ Thioridazine is less commonly prescribed today than it once was, but it remains on the market and in use, particularly in settings where newer antipsychotics are unavailable or have not worked.

Some Less Expected Medications

Beyond the well-known offenders, a few other medications turned up as significantly overrepresented among patients with unconfirmed PCP-positive screens. These included alprazolam (Xanax), clonazepam (Klonopin), and, more surprisingly, carvedilol, a beta-blocker used for heart failure and high blood pressure. Each of these medications was found in roughly 9 to 10% of patients whose PCP screens failed confirmation.⁴ Whether these drugs directly cause cross-reactivity or simply happened to be common in the study population is not entirely clear, but their overrepresentation was statistically significant enough to flag them as potential contributors.

Carvedilol is a particularly unexpected name on that list. It is a cardiac medication with no obvious structural or pharmacological relationship to PCP, and its presence raises the possibility that some false positives involve metabolic pathways or urinary byproducts that are not yet well understood. The benzodiazepines alprazolam and clonazepam are also surprising, since they belong to a completely different drug class. These findings reinforce how imprecise initial immunoassay screens can be.

Novel Ketamine Analogues

Ketamine is structurally related to PCP, and novel analogues of ketamine that have appeared on the recreational drug market can trigger false-positive PCP immunoassays.⁸ This matters for two reasons. First, someone who uses a ketamine-derived substance recreationally may face a PCP-positive screen even though they have never touched PCP itself. Second, legitimate medical use of ketamine and its derivative esketamine (Spravato, used for treatment-resistant depression) could plausibly produce the same cross-reactivity. If you are receiving ketamine infusions or nasal esketamine as a medical treatment, a PCP false positive is a realistic possibility you should discuss with whoever is ordering your drug screens.

What About Herbal Supplements

Given how many medications can interfere with PCP screens, you might wonder whether common herbal supplements cause similar problems. Research looking at ginkgo biloba, saw palmetto, St. John’s wort, ginseng, garlic, green tea, valerian, and cranberry found that none of them triggered false-positive results on urine drug screens.⁹ That is reassuring, though it does not cover every supplement on the market. If you take something obscure, the honest answer is that nobody has tested it yet.

Why Confirmatory Testing Exists

The entire reason the medical and legal systems distinguish between a “screening test” and a “confirmatory test” is that screening tests are known to have a significant false-positive rate. Screening immunoassays are designed to be sensitive, meaning they are set up to catch as many true positives as possible, even at the cost of also flagging some people who did not actually use the drug. When a screen comes back positive, the sample should then be tested using a more specific analytical method, typically gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-tandem mass spectrometry. These techniques identify compounds by their exact molecular structure rather than by antibody binding, which eliminates the cross-reactivity problem that causes false positives in the first place.

In practice, though, confirmatory testing is not always performed. In some emergency departments, clinical decisions are made based on the initial screen alone. In some workplace testing programs, the chain of custody and confirmation process may have gaps. And in some legal contexts, such as probation or child custody, a positive screen result can carry consequences before anyone thinks to request confirmation. If you receive a positive PCP screen and you have not used PCP, you have every reason to demand confirmatory testing before any decisions are made based on that result.

How to Protect Yourself

If you are taking any of the medications listed above and you know a drug test is coming, the simplest step is to provide a complete medication list to the testing facility ahead of time. Most reputable testing programs, especially those governed by federal workplace standards, have a Medical Review Officer whose job is to evaluate positive screens in the context of a person’s medication history before reporting a final result. Proactively disclosing your prescriptions and even your over-the-counter use gives that person the information they need to recognize a likely false positive.

If you are given a positive result without warning, ask specifically whether the test was a screening immunoassay or a confirmed result. If it was only a screen, request GC-MS or equivalent confirmation. Document everything: your prescriptions, your pharmacy records, any over-the-counter products you purchased recently. A positive PCP screen caused by your prescribed antidepressant or your nighttime cough medicine is not a reflection of drug use, and confirmation testing will demonstrate that clearly.

When the Test Kit Matters

Not all false positives are created equal across testing platforms. The lamotrigine false positive, for instance, was documented specifically on the Bio-Rad TOX/See system.⁶ The thioridazine case involved the EMIT II assay.⁷ The desvenlafaxine false positive was detected on a Medtox kit.² This means the problem is not just “what medications are you taking” but also “what test is the lab running.” Two hospitals in the same city may use different screening kits and get different results from the same urine sample.

For the average person, this is mostly out of your control. You are unlikely to know which assay platform a lab uses, and even if you did, the cross-reactivity profiles of specific kits are not published in consumer-friendly formats. What you can control is knowing which medications in your medicine cabinet are on the watch list, and being prepared to explain them if a test goes sideways.

The Psychiatric Patient’s Double Bind

There is a frustrating pattern buried in the data on PCP false positives. Many of the medications most likely to cause them, including venlafaxine, desvenlafaxine, lamotrigine, and thioridazine, are psychiatric medications. The people taking these drugs are already navigating the stigma of mental health treatment, and a false-positive PCP result can compound that stigma in settings where drug use is viewed with suspicion. In emergency departments, a positive PCP screen on a psychiatric patient may lead clinicians to attribute symptoms to substance use rather than to the underlying condition being treated. In forensic or legal settings, the consequences can be even more severe.

Case reports in the literature consistently flag this concern. A 17-year-old on long-term venlafaxine for anxiety, for example, had a false-positive PCP result that complicated their psychiatric assessment.¹ Clinicians and pharmacists who work with psychiatric populations need to be especially aware of these cross-reactivities, not just to avoid misdiagnosis but to avoid eroding trust with patients who are already in a vulnerable position. For patients themselves, knowing that your medication can cause this problem is a form of self-advocacy. You should not have to prove you are not using PCP because your antidepressant fooled a screening test.

1
PubMed Central. False-Positive Phencyclidine (PCP) Result on 11-Panel Urine Drug Screen (UDS) in a 17-Year-Old Adolescent with Long-Term Venlafaxine Use
2
PubMed Central. False-positive phencyclidine (PCP) on urine drug screen attributed to desvenlafaxine (Pristiq) use
3
Laboratory Medicine. Is Dextromethorphan a Concern for Causing a False Positive During Urine Drug Screening?
4
PubMed Central. How Often Do False-positive Phencyclidine (PCP) Urine Screens Occur with Use of Common Medications?
5
PubMed. False-positive urine phencyclidine immunoassay screen result caused by interference by tramadol and its metabolites
6
PubMed Central. Phencyclidine false positive induced by lamotrigine (Lamictal®) on a rapid urine toxicology screen
7
PubMed Central. Urine drug screens: Considerations for the psychiatric pharmacist
8
PubMed Central. Novel ketamine analogues cause a false positive phencyclidine immunoassay
9
US Pharmacist. Urine Drug Screening: Minimizing False-Positives and False-Negatives to Optimize Patient Care