Can Naltrexone Cause a False Positive Drug Test?

Naltrexone can cause a false positive on a urine drug screen, specifically on opioid panels that test for oxycodone. The culprit is not naltrexone itself but one of its breakdown products, a metabolite called noroxymorphone, which is structurally similar enough to oxycodone metabolites that standard immunoassay tests can mistake one for the other. This is an underappreciated problem for the thousands of people prescribed naltrexone to treat opioid or alcohol use disorders, since a positive opioid screen in that context can carry serious clinical, legal, and personal consequences.

How a Drug Meant to Block Opioids Looks Like One on a Test

The irony is hard to miss. Naltrexone is prescribed specifically to help people stop using opioids. It works by sitting on opioid receptors and blocking them, so even if someone takes an opioid, they feel little to no effect. Yet the same medication can make it look, on a standard urine drug screen, as though the person is actively using oxycodone.

The reason comes down to how the body processes naltrexone. When you take oral naltrexone, your liver breaks it down into several metabolites. The best-known one is 6-beta-naltrexol, but a lesser-known metabolite is noroxymorphone. That name might ring a bell: noroxymorphone is also a metabolite of oxycodone. So when a urine test is designed to flag the presence of oxycodone metabolites, noroxymorphone from naltrexone can trip the alarm. A published case report documented exactly this scenario in a patient stabilized on oral naltrexone who tested positive for oxycodone on a routine urine drug screen. Confirmatory testing revealed that the positive result was caused by noroxymorphone from naltrexone, not from any illicit opioid use.¹

Why Standard Urine Drug Screens Are Vulnerable

Most workplace, clinical, and criminal justice drug tests start with an immunoassay. These are the fast, relatively cheap tests that screen for broad classes of drugs rather than individual compounds. They use antibodies designed to bind to a target drug or its metabolites, but those antibodies are not perfectly selective. If another molecule has a similar enough shape, the antibody may latch onto it and register a positive result. This phenomenon, called cross-reactivity, is a well-documented limitation of immunoassay-based urine drug screening and a frequent source of false positives across many drug classes.²

For opioid panels, the antibodies are typically designed to detect compounds in the oxycodone or morphine family. Naltrexone belongs to a group of molecules called morphinans, which share a core chemical skeleton with drugs like morphine and oxycodone. The structural modifications that make naltrexone an antagonist (blocking opioid receptors instead of activating them) are enough to change its pharmacological effect completely, but they may not be enough to prevent every immunoassay antibody from recognizing its metabolites as opioid-class compounds.³

The practical upshot: a standard immunoassay cannot tell the difference between “this person is taking naltrexone as prescribed” and “this person recently used oxycodone.” That distinction only emerges with confirmatory testing.

What Confirmatory Testing Can and Cannot Do

When an initial screen comes back positive, the standard next step is confirmatory testing using a more precise analytical method, typically gas chromatography-mass spectrometry or liquid chromatography-tandem mass spectrometry. These techniques identify the exact molecules in a sample rather than relying on antibody cross-reactivity, so they can distinguish noroxymorphone produced from naltrexone metabolism from oxymorphone or other metabolites that would indicate actual oxycodone use.

In the documented case report, confirmatory testing identified noroxymorphone as the substance responsible for the positive oxycodone screen, ruling out use of oxycodone or other opioids.¹ The authors recommended that confirmatory testing should be standard practice whenever a naltrexone patient tests positive for opioids on an immunoassay screen.

The catch is that confirmatory testing is not always ordered automatically. In some settings, a positive immunoassay result is treated as the final word. Workplace drug testing programs, for instance, typically involve a Medical Review Officer who should follow up on positive screens, but the process depends on the individual program’s protocols and how well-informed the reviewing officer is about naltrexone’s metabolic profile. In clinical settings like addiction treatment programs, a positive opioid screen can trigger immediate changes to a patient’s treatment plan before confirmatory results come back. And in criminal justice contexts, a positive screen can lead to sanctions, revocation of probation, or loss of child custody. The gap between the initial positive screen and the confirmatory result is where real harm can happen.

Naltrexone Versus Naloxone

Naltrexone and naloxone are both opioid antagonists, and people frequently confuse them. They share a similar core structure and are both used in the treatment of opioid use disorder, though in different ways: naloxone is best known as an emergency rescue medication for overdose (often by the brand name Narcan), while naltrexone is taken daily or monthly to prevent relapse. Given their chemical similarities, you might expect both to cause false-positive opioid screens. But the evidence suggests they behave differently on this front.

A study specifically examined whether naloxone administration caused a positive urine opiate screen. In that study, all urine drug screens using the EMIT immunoassay technique came back negative for opiates at both dosage levels tested.⁴ This makes sense when you consider the metabolic differences between the two drugs. Naloxone is rapidly metabolized and cleared from the body, and its metabolic pathway does not produce noroxymorphone in meaningful quantities the way naltrexone’s does. Naltrexone, taken orally, undergoes extensive first-pass metabolism in the liver, producing a broader set of metabolites over a longer period.

So if you have been given naloxone during an overdose reversal or carry a naloxone auto-injector, a false-positive opioid screen from the naloxone itself is unlikely. The same reassurance does not apply to naltrexone.

Which Drug Tests Are Affected

Not every drug test is equally susceptible. The false-positive issue with naltrexone is specific to immunoassay-based urine drug screens, particularly those with oxycodone-specific panels. Older opiate panels that primarily target morphine and codeine may not cross-react with noroxymorphone in the same way, because their antibodies are calibrated to a slightly different molecular target. However, as oxycodone has become one of the most commonly abused prescription opioids, many modern immunoassay panels now include a separate oxycodone-specific channel, and it is this channel where naltrexone’s metabolite causes trouble.

Point-of-care rapid tests, the kind used in some doctor’s offices and addiction treatment clinics, are immunoassay-based and therefore susceptible to the same cross-reactivity. These cup-style or dipstick tests are convenient but are essentially performing the same antibody-binding reaction as a laboratory immunoassay, often with even less specificity.

Hair tests, oral fluid tests, and blood-based tests each have their own cross-reactivity profiles and are generally less likely to produce this specific false positive, though they are also less commonly used in routine screening. The vast majority of drug testing in the United States relies on urine immunoassays as the first step, which is why the naltrexone issue is clinically relevant.

Practical Steps If You Take Naltrexone

If you are prescribed naltrexone and know you will be subject to drug testing, a few steps can help protect you from the consequences of a false positive.

Disclose proactively: Tell the testing facility, your employer’s Medical Review Officer, or your treatment provider that you are taking naltrexone before the test, not after a positive result. Bringing it up after the fact can look like a convenient excuse, whereas bringing it up beforehand establishes a clear record.
Carry documentation: A prescription record or a letter from your prescribing physician confirming your naltrexone use can resolve questions quickly. Some people on naltrexone for addiction treatment are understandably private about that fact, but having documentation available prevents a far more damaging disclosure if a positive test triggers a formal investigation.
Request confirmatory testing: If a screen comes back positive, you have the right in most testing contexts to request that the sample undergo confirmatory analysis. In federally regulated workplace testing, confirmatory testing is mandatory before any action is taken. In other settings, you may need to specifically request it.
Know your program’s rules: Drug court programs, probation offices, and some employer-based testing programs have different rules about how positive screens are handled. Understanding those rules in advance lets you advocate for yourself if a false positive occurs.

The injectable form of naltrexone (sold as Vivitrol) delivers the same active drug and produces the same metabolites, so it carries the same false-positive risk as oral naltrexone. The route of administration does not eliminate the issue.

How Common Is This Problem

The honest answer is that nobody knows the exact frequency. The published evidence on naltrexone-specific false positives comes primarily from case reports and small case series rather than large epidemiological studies. This makes it difficult to say whether the false-positive rate is one in a hundred or one in a thousand naltrexone users. What is clear is that the mechanism is real, the metabolic pathway is well-established, and the problem has been documented in clinical practice more than once.

The under-reporting is easy to explain. When a person on naltrexone tests positive for opioids and the result is not sent for confirmatory testing, the false positive is never identified. It just looks like a relapse. In addiction treatment settings, a positive opioid screen in someone prescribed an opioid antagonist might be met with some suspicion, but if the clinician is unaware that naltrexone can produce this result, the false positive goes unrecognized. The patient may face consequences for a relapse that never happened.

This is one area where awareness has been slowly growing. The published case report explicitly calls for broader education of clinicians, testing laboratories, and criminal justice personnel about the possibility of naltrexone-induced false-positive oxycodone results.¹

The Broader False-Positive Landscape

Naltrexone is far from the only medication that causes false positives on drug screens. The cross-reactivity problem is widespread across immunoassay platforms and drug classes. Bupropion (an antidepressant also used for smoking cessation) is one of the most common culprits for false-positive amphetamine screens. In one study of over 10,000 urine drug screens, about a third of the unconfirmed amphetamine-positive results were linked to bupropion prescriptions.² Proton pump inhibitors like omeprazole have been associated with false-positive THC results. Certain antihistamines can mimic amphetamines. Sertraline, trazodone, and several other common psychiatric medications have documented cross-reactivity on various panels.

This broad false-positive problem is part of why confirmatory testing exists in the first place. Immunoassays are designed to cast a wide net, catching true positives at the cost of some false positives. The system only works correctly when the confirmatory step is consistently applied. When it is skipped or delayed, people taking legitimate medications pay the price.

Low-Dose Naltrexone and Off-Label Uses

In recent years, low-dose naltrexone (LDN) has gained popularity as an off-label treatment for conditions ranging from chronic pain to autoimmune disorders. Typical LDN doses are much lower than the standard addiction-treatment dose. The standard oral dose for opioid use disorder is 50 mg daily, while LDN usually falls in the range of 1 to 5 mg daily.

Whether LDN produces enough noroxymorphone to trigger a false-positive oxycodone screen is not well studied. In theory, the lower dose means less metabolite production, but immunoassay sensitivity thresholds vary by manufacturer and platform, and even a small amount of cross-reactive metabolite could potentially register as positive on a sensitive assay. If you are taking LDN and subject to drug testing, the same precautions apply: disclose the medication, carry documentation, and request confirmatory testing if a screen comes back positive. Assuming you are safe because the dose is low is a gamble without published data to back it up.

LDN is also frequently compounded by specialty pharmacies rather than dispensed as a commercially manufactured product, which can make documentation slightly more complicated. Having your prescriber provide a letter that specifies the medication, dose, and indication helps bridge that gap.

When the Test Result Changes Your Treatment

For people in addiction treatment, a false-positive opioid screen while on naltrexone creates a particularly painful scenario. The clinician may interpret it as evidence of relapse, which could lead to escalation of treatment intensity, changes in medication, loss of take-home dosing privileges, or discharge from a program. In drug court or probation settings, the consequences can be even more immediate: incarceration, community service, or loss of custody rights.

The patient, meanwhile, knows they have not used opioids but may struggle to be believed. People in recovery are often acutely aware of the stigma they face and the default assumption that any positive drug test means they are lying. A clinician who does not know about naltrexone’s metabolic quirk may interpret the patient’s protestations as denial, reinforcing a narrative that harms the therapeutic relationship.

This dynamic is exactly why the published research on this topic emphasizes clinician education. When prescribers, testing labs, and program administrators are aware that naltrexone can produce false-positive oxycodone results, they are more likely to order confirmatory testing before acting on a positive screen. That simple step, ordering confirmation before making clinical or legal decisions, prevents the cascade of consequences that a false positive can trigger.