Drug Interaction Report

Granisetron Transdermal Patch and Phenobarbital: Interaction Details

A pharmacist-reviewed analysis of the Granisetron Transdermal Patch and Phenobarbital 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

Phenobarbital

Fidoquel Sezaby
+

Granisetron Transdermal Patch

Sancuso Sancuso®
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 281 documented Granisetron Transdermal Patch interactions, 265 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
delayed
Evidence
probable
Severity
Major

What happens

An increased risk of QT interval prolongation and reduced granisetron exposure

Interaction Deep Dive

In a human pharmacokinetic study, coadministration of PHENobarbital and intravenous granisetron resulted in a 25% increase in the total plasma clearance of granisetron. While this increased clearance is attributed to PHENobarbital's induction of the cytochrome P450-mediated granisetron metabolism, the clinical significance of this interaction is unknown1. Also, avoid the concomitant use of PHENobarbital with other products with a known potential to prolong the QTc interval or products that may increase concentrations of PHENobarbital. PHENobarbital causes QTc interval prolongation. Concomitant use of PHENobarbital with other products that prolong the QTc interval like granisetron may result in a greater increase in the QTc interval and adverse reactions associated with QTc interval prolongation. If concomitant use of granisetron and PHENobarbital is unavoidable, monitor patients for increased risk of QTc interval prolongation 2.

Why it happens (mechanism)

Additive QT interval prolongation; induction of CYP3A-mediated metabolism of granisetron

Common questions

Can I take Granisetron Transdermal Patch and Phenobarbital together?

An increased risk of QT interval prolongation and reduced granisetron exposure Always confirm with your pharmacist or prescriber before making any change.

How serious is the Granisetron Transdermal Patch and Phenobarbital interaction?

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

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 Granisetron Transdermal Patch or Phenobarbital 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: KYTRIL(R) oral tablets, oral solution, granisetron hydrochloride oral tablets, oral solution. Roche Laboratories Inc, Nutley, NJ, 2005.
  2. Product Information: SEZABY(TM) intravenous injection, phenobarbital sodium intravenous injection. Sun Pharmaceutical Industries Inc (per manufacturer), Cranbury, NJ, 2022. DailyMed
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Phenobarbital

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Granisetron Transdermal Patch

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All Granisetron Transdermal Patch interactions (281) Granisetron Transdermal Patch drug information
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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:

Phenobarbital + 1,4-butanediol Activated Charcoal Beer Gamma-hydroxybutyrate (ghb) Henbane Kava L-tryptophan Scopolia + 166 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.