Diphenhydramine is the active ingredient in Benadryl and dozens of other over the counter allergy pills and sleep aids. It is also one of the most frequently documented sources of false positive results on urine drug screens. If someone took a normal dose of an antihistamine before a scheduled test and the screen came back non-negative for phencyclidine (PCP), methadone, or a tricyclic antidepressant (TCA), the medication is a real and published explanation, not an excuse to dismiss.
This article explains how that cross-reactivity happens, which panels are actually affected, what the published research shows, and what employers and medical review officers are supposed to do when it comes up. It is not a reason to skip confirmation testing, and it is not a way to talk anyone out of a positive result that GC-MS or LC-MS/MS has already confirmed.
How immunoassay screening actually detects cross-reactivity
The initial screen used on most urine drug test cups and cartridges is an immunoassay. It works by mixing the specimen with antibodies designed to bind to a target drug or its metabolites. If enough of the target binds, the strip reads negative; if too little binds because the antibody sites are occupied by something else, the strip reads non-negative. That "something else" does not have to be the actual drug of interest. It only has to be a molecule with a similar enough shape, or epitope, to fool the antibody.
Diphenhydramine has a molecular structure that happens to resemble several classes of controlled substances closely enough to bind those antibodies at high concentrations. This is documented cross-reactivity, not a lab error. The Substance Abuse and Mental Health Services Administration's own guidance to medical review officers acknowledges that immunoassay screens are subject to cross-reactivity from legal medications and that a non-negative screen is a preliminary result, not a final one.
Which panels diphenhydramine can affect
Not every panel is vulnerable. The cross-reactivity documented in the medical literature clusters around three analytes in particular.
| Panel | Documented diphenhydramine effect | What the evidence shows |
|---|---|---|
| PCP | Yes, well documented | A retrospective study of hospital urine screens found diphenhydramine was present in 14.8 percent of non-confirmed (false positive) PCP screens versus 9.5 percent of confirmed positives, and that 17.9 percent of all initial positive PCP immunoassay screens did not confirm on GC-MS. |
| Methadone (MTD) | Yes, documented in a pediatric case with lab confirmation | A published case showed a rapid urine screen reading positive for methadone after diphenhydramine ingestion, with no methadone exposure and a negative GC-MS. In vitro testing confirmed the parent diphenhydramine molecule, not its metabolites, cross-reacted with the methadone assay. |
| Tricyclic antidepressants (TCA) | Yes, documented in overdose-level case reports | A case report of diphenhydramine intoxication describes a positive TCA immunoassay that stayed positive for 60 to 90 hours before clearing, since diphenhydramine shares structural features with TCAs. |
| THC, opiates, cocaine, amphetamines, benzodiazepines | No credible cross-reactivity documented | These panels target different antibody families and are not implicated in the diphenhydramine literature reviewed here. |
The methadone and TCA cases in the literature involved fairly high doses, including one intentional overdose of 2,000 milligrams. The PCP data includes cases at more ordinary over the counter dosing. Either way, dose and individual metabolism both affect whether cross-reactivity shows up on a given screen, which is exactly why a screen alone is never treated as a final answer. For more on medications that interact with panels generally, see American Screening Corporation's overview of medications that can cause a false positive on drug tests, and for background on why PCP remains on standard panels despite this issue, see why PCP is still on drug test panels.
What the research actually shows
The strongest documentation comes from three sources. A hospital-based retrospective analysis of PCP immunoassay screens, published in Clinical Toxicology and archived on PubMed Central, found diphenhydramine was one of the medications most frequently associated with non-confirmed PCP results, though the association did not reach statistical significance across the full sample. The same study underscores that a meaningful share of all initial positive PCP screens, not just diphenhydramine-linked ones, failed to confirm on GC-MS, which is the core argument for never treating an immunoassay screen as final.
A separate case report describes a pediatric patient whose rapid urine screen was positive for methadone after diphenhydramine ingestion with no methadone exposure in the history. Laboratory confirmation was negative for methadone, and in vitro testing traced the cross-reactivity specifically to the parent diphenhydramine compound.
A follow-up review of two diphenhydramine overdose cases reached the same conclusion, describing false positive TCA results that persisted for days before clearing, illustrating that this is not a momentary glitch but something that can outlast a person's memory of when they took the medication. A broader clinical review of PCP intoxication in the emergency medicine literature likewise lists diphenhydramine among the substances known to interfere with PCP immunoassays.
Why confirmation testing resolves the question
This is the part that matters most for anyone relying on a drug testing program. Immunoassay screens are fast and inexpensive because antibody binding is a simple reaction, but that same simplicity is what allows cross-reactivity to happen. Confirmatory testing works differently. Gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) separate and identify molecules by their actual mass and structure rather than by antibody binding. Diphenhydramine does not have the same mass fragmentation pattern as PCP, methadone, or a tricyclic antidepressant, so confirmatory methods correctly distinguish it every time.
Under the federal Mandatory Guidelines for Federal Workplace Drug Testing Programs, any non-negative immunoassay result on a regulated test must go to confirmatory GC-MS or LC-MS/MS testing before it can be reported as positive, and the same two-step model (screen, then confirm) is standard practice across most non-regulated workplace and clinical programs as well. A screen result by itself is a preliminary, presumptive finding, never a diagnosis and never proof of drug use.
What a Medical Review Officer does with a disclosed OTC medication
On DOT-regulated and most other regulated testing programs, a certifying scientist reviews any confirmed non-negative result before it reaches a Medical Review Officer (MRO), a licensed physician trained to interpret drug test results. Under 49 CFR Part 40, the MRO contacts the individual who tested non-negative, discusses the confirmed result, and asks about legitimate medical explanations, including prescription and over the counter medications. If a person reports taking diphenhydramine and the confirmed lab result is consistent with a documented cross-reactive medication rather than an actual controlled substance, the MRO has a defined process for evaluating that explanation against the confirmatory chemistry, not against the person's word alone.
This is also why an initial screen that shows non-negative should never be treated as a positive result on its own. The screen flags a specimen for further testing. It is the confirmed laboratory result, evaluated against the individual's medical history by a qualified MRO, that determines the final verified outcome.
What employers and testing programs should do
The practical takeaway for anyone running a testing program is straightforward. Never take adverse action, including termination, discipline, or a rescinded job offer, based on an unconfirmed immunoassay screen alone. Build confirmation testing into the program for any non-negative result before it is treated as a final finding, and give the tested individual a documented opportunity to disclose medications before or during MRO review. State laws on drug testing procedure and employee rights vary, so employers should confirm their obligations under their own state's rules in addition to any federal framework that applies to their industry.
Programs that rely on point of care immunoassay drug testing cups for the initial screening step should pair that step with a clear, written policy for routing every non-negative result to laboratory confirmation, and for documenting the MRO or reviewing physician's disposition of the case. That combination, a reliable screening cup plus a defined confirmation and review pathway, is what makes a testing program legally defensible when a cross-reactive medication like diphenhydramine shows up.
Frequently asked questions
Can taking Benadryl actually cause a positive drug test?
It can cause a non-negative immunoassay screen for PCP, methadone, or tricyclic antidepressants in some individuals, according to published case reports and a hospital-based retrospective study. It does not cause a true positive on a confirmatory GC-MS or LC-MS/MS test, because those methods identify the actual molecule rather than relying on antibody binding.
How long can a diphenhydramine-related false positive last?
Case reports describe a false positive TCA immunoassay result persisting for 60 to 90 hours after a large diphenhydramine overdose before clearing. Ordinary over the counter dosing has not been studied in the same detail, and individual results will vary with dose and metabolism.
Does this mean employers should ignore a non-negative screen if someone says they took Benadryl?
No. Every non-negative screen on a defensible testing program should go through confirmatory laboratory testing and Medical Review Officer evaluation regardless of what the individual reports. The disclosure is information for the reviewing physician to weigh against the confirmed lab chemistry, not a reason to skip confirmation.
Which drug test panels are most affected by diphenhydramine?
The documented cross-reactivity is concentrated on PCP, methadone, and tricyclic antidepressant immunoassays. THC, standard opiate, cocaine, amphetamine, and benzodiazepine panels are not implicated in the published cross-reactivity literature.
Is it safe to stop taking an antihistamine before a scheduled drug test?
This article is not medical advice and does not recommend starting, stopping, or adjusting any medication. Anyone with questions about their own medication and an upcoming test should talk to their prescribing clinician or the Medical Review Officer handling their test.
This article is general information, not medical or legal advice. It does not recommend starting, stopping, or adjusting any medication, and it is not intended to help anyone influence a drug test result. Testing programs should rely on confirmatory laboratory testing and Medical Review Officer review before treating any screening result as final, and should check their own state laws on drug testing procedure.
