Phenytoin, sold under the brand name Dilantin, is a hydantoin anticonvulsant used to control seizures. It is not a controlled substance and it is not an opioid, a stimulant, or a benzodiazepine. The question employers and employees ask is narrower and more specific: because phenytoin shares part of its chemical structure with the barbiturate family, can it make a urine screen read positive on the barbiturate (BAR) panel. This article looks at what the drug label actually says about that chemical relationship, what a BAR immunoassay is designed to detect, and why a laboratory confirmation test resolves the question every time it matters.
What phenytoin is and why the question comes up
Phenytoin belongs to the hydantoin class of anticonvulsants, a chemical family that is related to, but distinct from, the barbiturate class. A 2024 peer reviewed pharmacology review of antiepileptic drug monitoring groups phenytoin and mephenytoin together as hydantoins, while placing phenobarbital, mephobarbital, and primidone in the separate barbiturate group, each with its own ring structure and metabolic pathway, as described in the review of antiepileptic drug determination methods indexed in PubMed Central. The two classes are close cousins on paper, which is exactly why the barbiturate question keeps surfacing for a drug that is not itself a barbiturate.
The manufacturer's own prescribing information settles the chemistry question directly. The current Dilantin label states that the drug "is related to the barbiturates in chemical structure, but has a five membered ring" rather than the six membered ring found in true barbiturates, a distinction confirmed on the DailyMed label for Dilantin Infatabs. That single structural difference, a five membered imidazolidinedione ring instead of a six membered barbituric acid ring, is the reason phenytoin is classified as a hydantoin and not a barbiturate, even though the family resemblance is real.
Screening immunoassay versus confirmation
A workplace urine cup or dip card BAR panel does not identify a molecule by name. It uses antibodies raised against phenobarbital or a closely related barbiturate to detect a shape in the sample. If a different molecule happens to fit that antibody well enough, the screen can read non negative even when no actual barbiturate is present. This is called cross reactivity, and it is a known limitation of every immunoassay screen, not just the BAR panel. A screening result is a presumptive flag, never a confirmed finding on its own.
That is why every credible drug testing program, and every reputable collection kit, pairs the immunoassay screen with a confirmation step on any non negative result. Gas chromatography mass spectrometry (GC/MS) or liquid chromatography tandem mass spectrometry (LC/MS) separates compounds by their exact mass and fragmentation pattern rather than by antibody shape. Phenytoin and phenobarbital have different molecular weights and different fragmentation signatures, so a properly run confirmation test distinguishes the two every time, regardless of what the initial screen showed. Panels built into drug test cups and dip cards, including the multi panel drug test cups in this collection, are designed as the presumptive first step precisely because that second, definitive step exists.
What the evidence actually shows about phenytoin and BAR screens
Here is where it is important to be precise rather than alarmist. The documented fact is the structural relationship described on the Dilantin label above. What is not available in current, freely accessible primary literature is a well replicated, modern data set quantifying how often phenytoin triggers a false positive on today's competitive immunoassay BAR screens used in workplace testing. Older case literature and laboratory manuals have long flagged hydantoins as a theoretical interferent class worth watching because of the shared ring chemistry, but that is a structural caution, not a documented epidemic of false positives. The honest answer for an employer or an employee is that the evidence for cross reactivity is limited and dated rather than robust, and the correct response to any non negative BAR screen is the same regardless: send it to confirmation and let the laboratory result, not the screen, decide the outcome.
What phenytoin definitely does not do is show up on the opiate, amphetamine, cocaine metabolite, THC, or PCP panels. Its structural overlap is specific to the barbiturate class because of the shared ring system. An employer who sees a non BAR panel flag on an employee taking only phenytoin is looking at an unrelated cause that has nothing to do with this medication.
Phenobarbital is the real target of the BAR panel
It helps to separate the two drugs clearly. Phenobarbital is a true barbiturate, and it is exactly what the BAR immunoassay is built to detect, as its own current prescribing label confirms on DailyMed's phenobarbital tablet label. Primidone, another anticonvulsant, is metabolized in the body into phenobarbital, so a person taking primidone can produce a genuine, pharmacologically expected BAR positive that confirmation will verify as real. Phenytoin is neither of those things. For a closer look at how the panel is built around phenobarbital itself, see this site's separate piece on the barbiturate panel and how phenobarbital is detected and confirmed.
Anticonvulsants and documented screening interference
| Medication | Chemical class | Panel it can affect | What confirmation shows |
|---|---|---|---|
| Phenytoin (Dilantin) | Hydantoin | Barbiturate (BAR), structural caution only | GC/MS or LC/MS separates it from phenobarbital; not itself a barbiturate |
| Phenobarbital | Barbiturate | Barbiturate (BAR) | Confirms as a true, expected positive for a prescribed user |
| Primidone | Barbiturate prodrug | Barbiturate (BAR) | Metabolizes to phenobarbital; confirms as a true positive |
| Carbamazepine | Iminostilbene | Tricyclic antidepressant (TCA) screens, reported in case literature | Mass spectrometry separates it from true TCAs |
| Valproic acid | Carboxylic acid derivative | No well documented panel interference | Not an expected screen interferent |
No dosing or medical advice, only documentation
Nothing in this article is a reason to change, skip, or adjust a prescribed dose of phenytoin or any other anticonvulsant around a scheduled drug test. Seizure medication decisions belong to the prescribing physician. The only action an employee needs to take is administrative: bring current prescription documentation, keep the pharmacy label or a note from the prescriber on hand, and be ready to provide that information if a medical review officer calls. Hiding or delaying this information only slows down a process that is designed to resolve quickly when the paperwork is available.
The medical review officer's role
A medical review officer, or MRO, is a licensed physician trained to review laboratory confirmed non negative results, contact the employee, ask about current prescriptions and legitimate medical explanations, and decide whether a result should be reported to the employer as positive or as negative based on a verified medical explanation. In federally regulated safety sensitive testing, 49 CFR Part 40 Subpart G spells out exactly what an MRO must do before any non negative result reaches an employer, including the interview process and the standards for accepting a medical explanation, as set out in the full text of 49 CFR Part 40 on GovInfo. Outside DOT covered testing, employers generally build their own programs around the same MRO concept, and the Substance Abuse and Mental Health Services Administration maintains public background on how that review process and panel selection work on its drug testing resources page. This site also has a dedicated explanation of how the MRO verification process works step by step for employers setting up a program.
An employer should never try to interpret a screening result on its own or ask an employee to explain a non negative result informally. That conversation belongs to the MRO, who has the training and the legal standing to request documentation, verify it with a pharmacy or prescriber, and make a defensible call. Employers may generally rely on the MRO's verified result as the record of what happened, but should consult counsel on how to apply any particular outcome to employment decisions, since program design and state rules vary.
What this means in practice
If an employee taking prescribed phenytoin receives a non negative screen on a BAR panel, the correct sequence is simple. The screen goes to confirmation automatically in any properly run program. Confirmation by GC/MS or LC/MS will show whether phenobarbital, a genuine barbiturate, is actually present, or whether the screen reacted to something else entirely, including the structural similarity between hydantoins and barbiturates described on the Dilantin label. The MRO reviews the confirmed result alongside the employee's prescription information and issues a verified outcome. At no point does an employer, a collector, or the employee need to guess at the chemistry. The system is built so that a documented prescription and a confirmation test answer the question together.
Frequently asked questions
Does phenytoin cause a false positive on a drug test?
Phenytoin is not a controlled substance and is not routinely screened for on its own. It is chemically related to barbiturates, which has led to it being discussed as a possible interferent on barbiturate (BAR) immunoassay screens, but current, freely available evidence for that effect is limited rather than well documented. Any non negative screen should go to confirmation and MRO review before any conclusion is drawn.
Is Dilantin a controlled substance?
No. Phenytoin, sold as Dilantin, is a prescription anticonvulsant and is not scheduled as a controlled substance.
Why does phenytoin sometimes get flagged on a barbiturate screen?
Phenytoin is a hydantoin, a chemical class related to barbiturates. Its label describes it as related to barbiturates in structure but built on a five membered ring rather than the six membered ring of true barbiturates, which is the basis for any screening cross reactivity discussion.
Can a lab confirmation test tell phenytoin apart from phenobarbital?
Yes. GC/MS and LC/MS confirmation identify compounds by exact mass and fragmentation pattern, not by antibody shape, so phenytoin and phenobarbital are reliably distinguished at the confirmation step even when a screening immunoassay cannot tell them apart.
What should an employee taking phenytoin do before a drug test?
Bring current prescription documentation or a note from the prescribing physician and be ready to provide it to the medical review officer if contacted. No dose change or other medical action is needed for testing purposes.
Does a prescription for phenytoin need to be reported to the MRO?
Only if a drug test comes back non negative and the MRO contacts the employee as part of the verification interview. At that point, current and accurate prescription information allows the MRO to complete the review and issue a verified result.
This article is general information for employers and employees about drug testing science and process. It is not legal or medical advice, and it is not a substitute for guidance from a medical review officer, a prescribing physician, or legal counsel.