Drug Interaction Report

Ketoconazole Topical and Fesoterodine: Interaction Details

A pharmacist-reviewed analysis of the Ketoconazole Topical and Fesoterodine 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

Fesoterodine

Toviaz
+

Ketoconazole Topical

Extina® Foam Ketozole® Cream Kuric Nizoral AD® Shampoo Nizoral® Cream Nizoral® Shampoo Xolegel®
Dr. Brian Staiger, PharmD, BCPS
Medically reviewed by
Dr. Brian Staiger, PharmD, BCPS
Updated Aug 8, 2026
Editorial policy · Data sources
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Interaction severity
Major
Potentially serious — often needs a change or close monitoring.
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 546 documented Ketoconazole Topical interactions, 380 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
theoretical
Severity
Major

What happens

Increased fesoterodine active metabolite exposure

Interaction Deep Dive

Use caution when oral ketoconazole is coadministered with fesoterodine. Upon coadministration, monitor patients closely for signs or symptoms of increased or prolonged effects or side effects of fesoterodine, and reduce dosage of fesoterodine as deemed necessary1. Concomitant use of fesoterodine and a strong CYP3A4 inhibitor, such as ketoconazole may result in increased exposure of the active metabolite of fesoterodine. If fesoterodine and ketoconazole are coadministered, do not exceed fesoterodine 4 mg daily in adults and pediatric patients 6 years or older and weighing greater than 35 kg. Coadministration in children less than 35 kg is not recommended 2.

Why it happens (mechanism)

Inhibition of CYP3A4-mediated metabolism of the active metabolite of fesoterodine

Literature reports

1 report — tap to read

a) In 2 studies, coadministration of ketoconazole 200 mg twice daily for 5 days given with fesoterodine 8 mg has resulted in increased Cmax of the active fesoterodine metabolite by 1.5 and 2.1-fold (poor CYP2D6 metabolizers) and 2.2-fold (extensive CYP2D6 metabolizers), while AUC increased 1.9 and 2.5-fold (poor CYP2D6 metabolizers) and 2.2-fold (extensive CYP2D6 metabolizers) 2.

Common questions

Can I take Ketoconazole Topical and Fesoterodine together?

Increased fesoterodine active metabolite exposure Always confirm with your pharmacist or prescriber before making any change.

How serious is the Ketoconazole Topical and Fesoterodine 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 "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.

Questions for your pharmacist

  • Does my dose of Ketoconazole Topical or Fesoterodine 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 (2)

  1. Product Information: Ketoconazole oral tablet, ketoconazole oral tablet. Taro Pharmaceuticals U.S.A. Inc. (per DailyMed), Hawthorne, NY, 2024. DailyMed
  2. Product Information: TOVIAZ(R) oral extended-release tablets, fesoterodine fumarate oral extended-release tablets. Pfizer Labs (per FDA), New York, NY, 2024. DailyMed
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