Drug Interaction Report

Mometasone and Phenytoin: Interaction Details

Name: Mometasone and Phenytoin Interaction
Creator: Dr. Brian Staiger, PharmD, BCPS

A pharmacist-reviewed analysis of the Mometasone and Phenytoin interaction: the documented clinical effects, severity rating, onset, mechanism, and management considerations, with cited references throughout.

AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

Phenytoin

Dilantin Phenytek

Mometasone

Asmanex Nasonex Propel Sinuva

Medically reviewed by

Dr. Brian Staiger, PharmD, BCPS

Updated Aug 8, 2026

Editorial policy · Data sources

Interaction severity

Major

Potentially serious — often needs a change or close monitoring.

Minor

Moderate

Major

Contraindicated

How we grade severity & evidence

Severity levels

Contraindicated: These should generally not be used together.
Major: Potentially serious — often needs a change or close monitoring.
Moderate: Can be significant — usually manageable with monitoring.
Minor: Usually limited clinical impact.

Evidence grades

Established: Well documented — supported by controlled studies or strong clinical data.
Probable: Good supporting evidence, though not definitively proven.
Suspected: Some evidence suggests this interaction, but it is not well established.
Possible: Limited or conflicting evidence; the interaction may occur.
Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.

Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.

Of 45 documented Mometasone interactions, 24 are rated major — including this one.

Worried about symptoms right now? Contact your pharmacist or prescriber, or call Poison Control at 1-800-222-1222 (US). Call 911 for an emergency.

Onset

unspecified

Evidence

probable

Severity

Major

What happens

Reduced corticosteroid exposure

Interaction Deep Dive

Concurrent administration of phenytoin or fosphenytoin (prodrug of phenytoin) and corticosteroids have resulted in reduced corticosteroids exposure. Consider corticosteroids dose adjustments to achieve optimal clinical response when phenytoin or fosphenytoin is added to or withdrawn from a patient's regimen¹²³.

Why it happens (mechanism)

Induction of hepatic drug-metabolizing enzymes by phenytoin

Literature reports

3 reports — tap to read

a) Concomitant methylPREDNISolone and phenytoin therapy has been reported to enhance the metabolism of the corticosteroid probably as a result of microsomal liver enzyme induction by phenytoin. One study stated the baseline half-life was decreased by 50% in patients on concurrent therapy ⁴.

b) Phenytoin has been demonstrated to increase the hepatic metabolism of corticosteroids. Those steroids with a pronounced effect include dexAMETHasone, fludrocortisone, methylPREDNISolone, and predniSONE ⁵⁶⁷.

c) Phenytoin has also been reported to reduce the therapeutic effects of prednisoLONE ⁸.⁹.

Common questions

Can I take Mometasone and Phenytoin together?

Reduced corticosteroid exposure Always confirm with your pharmacist or prescriber before making any change.

How serious is the Mometasone and Phenytoin interaction?

It is rated major. Potentially serious — often needs a change or close monitoring.

How quickly could this interaction happen?

The documented onset is "unspecified". The timing of this interaction is not well characterized.

How strong is the evidence for this interaction?

The evidence is graded "probable". Good supporting evidence, though not definitively proven.

Questions for your pharmacist

Does my dose of Mometasone or Phenytoin need adjusting while I take them together?
What symptoms should prompt me to call you or my prescriber right away?
Does the timing of my doses matter for this combination?
Is there a safer alternative to one of these medications for me?

References (9)

Product Information: FOSPHENYTOIN SODIUM intravenous and intramuscular injection, fosphenytoin sodium intravenous and intramuscular injection. SAGENT Pharmaceuticals (per Dailymed), Schaumburg, IL, 2024. DailyMed
Product Information: PHENYTOIN SODIUM intravenous and intramuscular injection, phenytoin sodium intravenous and intramuscular injection. Acella Pharmaceuticals, LLC (per Dailymed), Alpharetta, GA, 2024. DailyMed
Product Information: PHENYTOIN SODIUM oral extended-release capsules, phenytoin sodium oral extended-release capsules. Sun Pharmaceutical Industries, Inc. (per Dailymed), Cranbury, NJ, 2023. DailyMed
Stjernholm MR & Katz FH: Effects of diphenylhydantoin, phenobarbital, and diazepam on the metabolism of methylprednisolone and its sodium succinate. J Clin Endocrinol Metab 1975; 41:887-893. PubMed
Keilholz U & Guthrie GP Jr: Adverse effect of phenytoin on mineralocorticoid replacement with fludrocortisone in adrenal insufficiency. Am J Med Sci 1986; 291:280-283.
Petereit LB & Meikle AW: Effectiveness of prednisolone during phenytoin therapy. Clin Pharmacol Ther 1977; 22:912-916. PubMed
McLelland J & Jack W: Phenytoin/dexamethasone interaction: a clinical problem (letter). Lancet 1978; 1:1096-1097. DOI
Gambertoglio JG, Holford NH, Kapusnik JE, et al: Disposition of total and unbound prednisolone in renal transplant patients receiving anticonvulsants. Kidney Int 1984; 25:119-123. PubMed
Wassner SJ, Pennisi AJ, Malekzadeh MH, et al: The adverse effect of anticonvulsant therapy on renal allograft survival. A preliminary report. J Pediatr 1976; 88:134-137. DOI

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Phenytoin

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Mometasone

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All Mometasone interactions (45) Mometasone drug information

Also serious with: Mifepristone Desmopressin Desmopressin Nasal Atazanavir

Beyond drug–drug

These medications also interact with supplements

Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:

Phenytoin + 1,4-butanediol Activated Charcoal Beer Gamma-hydroxybutyrate (ghb) Kava L-tryptophan Siberian Cocklebur St. John's Wort + 216 more

Mometasone + Activated Charcoal Grapefruit St. John's Wort Acacia Rigidula Adrafinil African Wild Potato American Elder Astaxanthin + 133 more

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About our interaction database: depth, not breadth

HelloPharmacist doesn't license one of the big commercial interaction databases, so what you'll find here is not a complete list of every known drug interaction. Instead, each entry is researched, analyzed, and written up individually — documented from the published literature, reviewed by a pharmacist, and cited so you can check every source yourself.

If a combination isn't listed here, it means we haven't documented it yet — never that it's safe. For screening a full medication list, we recommend pairing this site with a comprehensive tool like the Drugs.com Interaction Checker and, most importantly, a review by your own pharmacist. For a deep, plain-English, fully sourced explanation of a specific documented interaction — that's exactly what we built this for.

This information is for education, not a substitute for professional medical advice. Do not start, stop, or change any medication without talking to your pharmacist or prescriber.