Drug & supplement interaction

Thiamine + Phenytoin

Phenytoin brand names: Dilantin Kapseals, Dilantin, Di-Phen, Dilantin Infatabs, Phenytek, and others.

No Known Interaction
Medically reviewed by · Updated July 2026 · Pair last verified Sep 20, 2026
The bottom line No known interaction

We found no known interaction between Thiamine and Phenytoin in our sources. Absence of a documented interaction is not the same as proven safety, and does not guarantee one does not exist. Do not start, stop, or change a medication or supplement based on this page; confirm with your pharmacist or doctor first. Additionally, although there is no interaction, the two are separately associated with some of the same side effects, which would make a new symptom harder to attribute to one or the other.

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Interaction report

Thiamine and Phenytoin

We checked these two against our interaction database. Here is what we found.

No Known Interaction Found

We did not find a documented interaction between Phenytoin and Thiamine in our sources. This does not guarantee that the combination is completely free of risk — there is often limited research on supplements and herbal products, especially around drug interactions.

If you decide to take both, the practical routine is simple: add one new thing at a time and note the date you started it. If the combination is going to disagree with you, it usually shows in the first days to a couple of weeks — and a start date turns “I’ve been feeling off” into something a pharmacist can actually work with. Do not stop a prescribed medicine on your own.

Always talk with your pharmacist or doctor before making changes to your medications or supplements, particularly if you take several medications or have an ongoing health condition.

Thiamine safety at a glance

Thiamine overall Generally safe Thiamine is generally very safe when taken by mouth in normal amounts.
Pregnancy Likely Safe When used orally in dietary amounts of 1.4 mg daily. A tolerable upper intake level (UL) has not been established for healthy individuals. Read the full pregnancy detail
Breastfeeding Likely Safe When used orally in dietary amounts of 1.4 mg daily. A tolerable upper intake level (UL) has not been established for healthy individuals. Read the full breastfeeding detail

Pregnancy & breastfeeding ratings are from Natural Medicines (Therapeutic Research Center) — the full detail is in the Special populations card below. Safety guidance is general; always confirm with your pharmacist or doctor for your situation.

Still wondering about combining Phenytoin and Thiamine? Ask Dr. Brian Staiger, PharmD, BCPS directly ›
How we checked

Why No Interaction Is Listed for Phenytoin

Interactions in our licensed database are recorded against drug classes, not just single drug names. Phenytoin is filed under 14 classifications — including cytochrome p450 2c8 (cyp2c8) substrates, cytochrome p450 2c9 (cyp2c9) inducers, anticonvulsants. Thiamine has documented interactions with 1 drug class — the most serious being trimethoprim (proloprim) (minor) — and none of Phenytoin’s classifications are among them.

Classification data comes from our licensed Natural Medicines dataset. A medication can belong to classes beyond those recorded there, so this describes how the check was run — it is not a safety guarantee.

Keep it in perspective

Thiamine Does Interact With Other Medications

Thiamine has 3 minor known drug interactions in our database — Phenytoin isn’t one of them. If you take anything else, check it against the full list below.

Minor 3 No known interaction 2,827 (incl. Phenytoin)
Taking anything else?

Check Another of Your Medications Against Thiamine

Covers the 2,830 medications we’ve checked against Thiamine. Pick one to open its full report.

What to actually watch for

No Interaction Found — But Data Suggests Thiamine and Phenytoin May Share Several of the Same Side Effects

This is not an interaction. Neither one changes how your body handles the other, and nothing here says they react together. What our sources do show is that each has separately been linked to the effects below — so taking both can make those effects more likely to turn up, and harder to pin on one or the other if they do. 3 of them are worth acting on rather than waiting out — marked below.

  • A severe allergic reaction / Hives / An allergic reaction Worth acting on Additional detail

    Thiamine

    “Orally, thiamine might rarely cause dermatitis and other allergic reactions. Parenterally, thiamine can cause anaphylactoid and hypersensitivity reactions, but this is also rare (”

    Thiamine · Natural Medicines adverse-effect record, reported in acute respiratory distress syndrome (ards).

    4 sources cited
    1. 15. McEvoy GK, ed. AHFS Drug Information. Bethesda, MD: American Society of Health-System Pharmacists, 1998.
    2. 35585. Rogovik, A. L., Vohra, S., and Goldman, R. D. Safety considerations and potential interactions of vitamins: should vitamins be considered drugs? Ann.Pharmacother. 2010;44(2):311-324. PubMed
    3. 105445. Thiamine hydrochloride injection package insert. Lake Zurich, IL: Fresenius Kabi, LLC; September 2019.
    4. 91170. Arruti N, Bernedo N, Audicana MT, Villarreal O, Uriel O, Muñoz D. Systemic allergic dermatitis caused by thiamine after iontophoresis. Contact Dermatitis. 2013 Dec;69(6):375-6. PubMed

    Phenytoin

    Listed in the ADVERSE REACTIONS section of the FDA label for Phenytoin. Read the label on DailyMed ›

  • A skin rash Additional detail

    Thiamine

    Recorded for Thiamine in the same Natural Medicines adverse-effect record shown under A severe allergic reaction / Hives / An allergic reaction above.

    Phenytoin

    Listed in the ADVERSE REACTIONS section of the FDA label for Phenytoin. Read the label on DailyMed ›

Reported for Thiamine but not commonly for Phenytoin: bleeding in the gut, shortness of breath, low blood pressure and numbness or tingling.

Effects for Phenytoin come from the ADVERSE REACTIONS section of its FDA label; effects for Thiamine come from the Natural Medicines adverse-effect record, quoted above. Wording is matched across sources by our own clinical vocabulary, so treat this as a prompt to pay attention, not a diagnosis.

Special populations

Thiamine in Pregnancy & Breastfeeding

Pregnancy And Lactation Likely Safe

When used orally in dietary amounts of 1.4 mg daily. A tolerable upper intake level (UL) has not been established for healthy individuals.

DISCLAIMER: This tool is intended for informational purposes only, and should not be interpreted as specific medical advice. Patients should consult with a qualified healthcare provider before making decisions about therapies and/or health conditions.© 2026 Therapeutic Research Center

Worth knowing

No Interaction — But Phenytoin May Lower Your Thiamine Levels

These two do not interact. The relationship runs the other way: the licensed record lists Phenytoin as associated with lower Thiamine levels — rated “Insufficient Evidence”.

Theoretically, phenytoin might reduce thiamine levels. Some evidence suggests that chronic treatment with phenytoin might reduce levels of thiamine in plasma and cerebrospinal fluid, and reduce uptake of thiamine into nerve tissue. Theoretically, this could contribute to side effects of phenytoin such as polyneuropathy and cerebellar ataxia. However, it isn't clear whether affected patients also had other factors contributing to thiamine deficiency, such as dietary insufficiency. There isn't enough evidence to recommend routine thiamine supplementation.

THIAMINE — Insufficient Evidence

See every medication linked to lower Thiamine levels · See the full Phenytoin nutrient report

Disclaimer: This checker finds which nutrients might be depleted by prescription or over-the-counter medications. Each nutrient depletion issue is rated based on clinical significance. Use your own professional judgment prior to making clinical decisions.© 2026 Therapeutic Research Center

From our Q&A

Questions About Thiamine or Phenytoin

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Thiamine or Phenytoin, not necessarily both, but if one is close to what you were about to ask it will usually go further than a database entry can:

Not quite your question? Ask our pharmacists directly — it is free, there is no account required, and answers usually come back within 24 hours.

Everything else you take

See Every Supplement That Interacts With Phenytoin

This page covers Thiamine only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Phenytoin page lists 224 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: 1,4-butanediol (major), Activated Charcoal (major), Beer (major), Gamma-hydroxybutyrate (ghb) (major) and Kava (major).

By severity: 9 major · 187 moderate · 28 minor.

From our pharmacist’s overview on the Phenytoin page

“Phenytoin (Dilantin) is a seizure medicine with a narrow therapeutic index, meaning small changes in its blood level can bring seizures back or tip over into toxicity, so I treat its supplement list with real seriousness. The biggest group, 78 ingredients, piles on sedation, and alcohol, kava, and GHB all rate major for exactly that re…”

Reviewed by 2 · Jul 18, 2026

Open the Phenytoin interaction page

One more smart check

Taking Thiamine as Part of a Dietary Supplement Product? Check It for Interactions

We hold 3 real supplement products containing Thiamine, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Thiamine. Even though Thiamine itself has no known interaction with Phenytoin, another ingredient on your label might. Find your product and we’ll check every ingredient on its label:

How much people look this up

Where Thiamine and Phenytoin Sit Against Everything Else

Out of every supplement and medication people look up on HelloPharmacist, here is where these two land this week — and which way each has been moving.

Supplement Thiamine
Top 25%

#178 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Phenytoin
Top 25%

#557 of 2,830 medications looked up here in the last 7 days

29 days ago today

Position among everything looked up on this site, not a measure of how commonly either one is used or how important it is. A supplement can climb this list because of a news story. It counts every request to the page, automated ones included, and nothing here identifies anyone.

Where does our data come from?
  • Source: the Natural Medicines (TRC Healthcare) clinical database — the evidence-graded professional reference for herb & supplement interactions.
  • Citation library: 103,510 references behind our interaction content, 65,311 of them resolved to verified PubMed/DOI sources. See every Thiamine reference on this page ↓
  • Freshness: we automatically re-check this combination every week — this pair was last verified September 20, 2026. The Thiamine and Phenytoin records this page draws on were last updated July 10, 2026.
  • What “no known interaction” means: no documented interaction in the sources above — not a guarantee of safety. Supplements are studied far less than prescription drugs, so we say exactly what the evidence supports, and nothing more.
Data structure & sourcing designed and verified by .
Supplement

About Thiamine

What is Thiamine?

  • Preventing or treating thiamine deficiency
  • Beriberi
  • Wernicke-Korsakoff syndrome (alcohol-related)
  • Supporting energy and nerve function
  • General B-vitamin supplementation

Thiamine (vitamin B1) is an essential nutrient your body needs to turn food into energy and to keep your nerves and heart healthy. Most people get enough from food, but supplements are clearly useful for deficiency, certain medical conditions, and people with heavy alcohol use. Talk with your pharmacist or doctor before starting it, especially if you have a chronic illness.

Tablets, capsules, oral liquids, injectable solutions (prescription), part of B-complex and multivitamins

An interaction check asks whether these two react with each other — it does not ask whether Thiamine is a good idea for someone in your situation. Here is what our Thiamine record says about it on its own.

Thiamine: dosing

The right dose of thiamine depends on your age, health, and the reason you are taking it. There is a recommended daily intake for general nutrition, and higher amounts are sometimes used by doctors to treat a deficiency.For supplements, the best approach is to follow the directions on the product label and not exceed them without guidance.

From our Thiamine monograph — the full record covers uses, side effects and every documented interaction.

Read the full Thiamine monograph & all interactions
Medication

About Phenytoin

What Is Phenytoin Used For?

Brand names: Dilantin Kapseals, Dilantin, Di-Phen, Dilantin Infatabs, Phenytek, Dilantin injection

Phenytoin is used to treat and prevent seizures. The specific types covered depend on the formulation. Oral extended-release capsules (including Phenytek), chewable tablets (including Dilantin Infatabs and Phenytoin Infatabs), and oral suspension (including Dilantin-125) are used for tonic-clonic (grand mal) and psychomotor (temporal lobe) seizures; most oral forms also cover prevention and treatment of seizures occurring during or after neurosurgery. Chewable tablets and some capsule formulations also cover complex partial seizures.

Parenteral (injectable) phenytoin sodium is used for generalized tonic-clonic status epilepticus (a prolonged, life-threatening seizure emergency) and for prevention and treatment of seizures during neurosurgery. The IV form can also substitute short-term for oral phenytoin when a patient cannot take it by mouth.

Most-Reported Side Effects for Phenytoin

From FAERS, the FDA’s public side-effect reporting system (40,278 reports, all brands). Reports are voluntary and are not proof the drug caused the effect.

Convulsion 3,522 reports
Seizure 3,095 reports
Toxicity to various agents 1,888 reports
Fall 1,604 reports
Dizziness 1,539 reports
See full Phenytoin drug information
Keep checking

Other Drugs & Thiamine

Browse every other medication we’ve checked against Thiamine — both the ones with a known interaction and the ones without. Select any drug for the full detail.

0Major
0Moderate
3Minor
2,826No known interaction
3 Minor 2,826 No interaction
Minor

3 interactions

No interaction 2,826 drugs checked — show the list

We also checked Thiamine against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 2,826 medications checked with no known interaction — see the full Thiamine list.

See the full Thiamine interactions page
See every supplement checked against Phenytoin, ranked by severity
Evidence & sources

Thiamine References

The 7 references that drive our Thiamine monograph and interaction data, from the evidence-graded Natural Medicines (TRC Healthcare) database. Citations with a link open the study on PubMed or the publisher’s site.

  1. McEvoy GK, ed. AHFS Drug Information. Bethesda, MD: American Society of Health-System Pharmacists, 1998.
  2. Yates AA, Schlicker SA, Suitor CW. Dietary reference intakes: The new basis for recommendations for calcium and related nutrients, B vitamins, and choline. J Am Diet Assoc 1998;98:699-706. PubMed
  3. Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline (2000). Washington, DC: National Academy Press, 2000. Available at: http://b
  4. Rogovik, A. L., Vohra, S., and Goldman, R. D. Safety considerations and potential interactions of vitamins: should vitamins be considered drugs? Ann.Pharmacother. 2010;44(2):311-324. PubMed
  5. Arruti N, Bernedo N, Audicana MT, Villarreal O, Uriel O, Muñoz D. Systemic allergic dermatitis caused by thiamine after iontophoresis. Contact Dermatitis. 2013 Dec;69(6):375-6. PubMed
  6. Thiamine hydrochloride injection package insert. Lake Zurich, IL: Fresenius Kabi, LLC; September 2019.
  7. Vora B, Wen A, Yee SW, et al. The Effect of Trimethoprim on Thiamine Absorption: A Transporter-Mediated Drug-Nutrient Interaction. Clin Pharmacol Ther 2023;114(2):381-392.

Medication overviews come from MedlinePlus (NIH); supplement label data from the NIH Dietary Supplement Label Database. Content is written and reviewed by licensed HelloPharmacist pharmacists — see our data sources and editorial standards.

Pharmacist Counseling Corner

Taking Thiamine and Phenytoin Together: Common Questions

Is it safe to take Thiamine with Phenytoin?
We do not list a known interaction between Thiamine and Phenytoin. That does not guarantee the combination is risk-free, so confirm with your pharmacist or doctor — especially if you take other medications or have a health condition.
Do Thiamine and Phenytoin have any of the same side effects?
Yes — our sources link each of them separately to 4 of the same effects. That is not an interaction: neither changes how your body handles the other. It does mean that if one of those effects turns up, it can be harder to tell which one is behind it. They are listed on this page with the source behind each.
Can Phenytoin lower my Thiamine levels?
Natural Medicines records Phenytoin as a medicine that can deplete Thiamine. That is the opposite direction to an interaction — it is a reason some people are told to take a supplement, not a reason to avoid one. Do not start one on the strength of this page; ask your pharmacist or doctor whether it applies to you.
How was the Thiamine and Phenytoin combination checked?
Interactions in our licensed database are recorded against drug classes. Phenytoin is filed under 14 classifications, and Thiamine has documented interactions with 1 drug class — none of Phenytoin's classifications are among them. A medication can belong to classes beyond those recorded in our dataset, so this describes how the check was run rather than guaranteeing safety.
Does Thiamine interact with any other medications?
Yes. Thiamine has 3 documented drug interactions in our database — Phenytoin is simply not one of them. If you take anything else, it is worth checking each one; they are all listed on this page.
Does Phenytoin interact with any supplements?
Yes. Phenytoin has documented interactions with 224 supplements in our database — Thiamine is not one of them. The Phenytoin interaction page linked on this page lists every one, ranked by severity.

Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy

Where next

Keep Exploring

Thiamine monograph Uses, safety profile, and every drug interaction in one place Phenytoin drug information The full plain-English guide to this medication Phenytoin supplement interactions Every supplement checked against Phenytoin, ranked by severity Supplement interaction checker Check any supplement or product against your medications Browse medications A–Z Every drug we’ve checked against herbs & supplements References The 7 citations behind this page Ask a pharmacist Free, personal answer from our licensed team

Still Have a Question About Phenytoin and Thiamine?

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This information is for education only and is not a substitute for professional medical advice. Always check with your pharmacist or doctor before starting, stopping, or combining supplements and medications.