Drug Interaction Report

Astemizole and Ketoconazole Topical: Interaction Details

A pharmacist-reviewed analysis of the Astemizole and Ketoconazole Topical 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

Ketoconazole Topical

Extina® Foam Ketozole® Cream Kuric Nizoral AD® Shampoo Nizoral® Cream Nizoral® Shampoo Xolegel®
+

Astemizole

No brand names on record
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
Contraindicated
These should generally not be used together.
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 63 documented Astemizole interactions, 1 is rated contraindicated — 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
delayed
Evidence
probable
Severity
Contraindicated

What happens

Increased astemizole exposure and an increased risk of QT interval prolongation

Interaction Deep Dive

The concurrent use of astemizole (CYP3A4 substrates that prolong QT) and ketoconazole is contraindicated as it may increase astemizole exposure and may also increase the risk of QT interval prolongation and sometimes resulting in life-threatening ventricular tachyarrhythmias including occurrences of torsades de pointes, a potentially fatal arrhythmia123. Enzyme inhibiting drugs such as ketoconazole may lead to high levels of astemizole if used concurrently. Astemizole overdoses have led to prolonged QT interval and severe ventricular arrhythmias 45.

Why it happens (mechanism)

Inhibition of CYP3A4-mediated metabolism of astemizole; additive QT interval prolongation

Literature reports

1 report — tap to read

a) Enzyme inhibiting drugs such as ketoconazole may lead to high levels of astemizole if used concurrently. Astemizole overdoses have led to prolonged QT interval and severe ventricular arrhythmias 45.

Common questions

Can I take Astemizole and Ketoconazole Topical together?

Increased astemizole exposure and an increased risk of QT interval prolongation Always confirm with your pharmacist or prescriber before making any change.

How serious is the Astemizole and Ketoconazole Topical interaction?

It is rated contraindicated. These should generally not be used together.

How quickly could this interaction happen?

The documented onset is "delayed". Effects tend to build up gradually over days to weeks.

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 Astemizole or Ketoconazole Topical 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 (5)

  1. Product Information: Ketoconazole oral tablet, ketoconazole oral tablet. Taro Pharmaceuticals U.S.A. Inc. (per DailyMed), Hawthorne, NY, 2024. DailyMed
  2. Anon: Reports of dangerous cardiac arrhythmias prompt new contraindications for the drug Hismanal. FDA Med Bull 1993; 23(1):2-3.
  3. Product Information: Hismanal(R), astemizole. Janssen Pharmaceutica Inc. Titusville, NJ, 1998.
  4. Hoppu K, Tapanainen P, Tikanoja T, et al: Accidental astemizole overdose in young children. Lancet 1991; 338:538-539. PubMed
  5. Snook J, Boothman BD, Watkins J, et al: Torsade de points ventricular tachycardia associated with astemizole overdose. Br J Clin Pract 1988; 42:257-259.
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Ketoconazole Topical

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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:

Astemizole + Activated Charcoal American Hellebore Arsenic Cesium Ephedra Grapefruit Sida Cordifolia Bitter Orange + 22 more

major · moderate · minor — check everything you take with our drug–supplement interaction checker.

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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.