Drug Interaction Report

Ethosuximide and Ritonavir: Interaction Details

Name: Ethosuximide and Ritonavir Interaction
Creator: Dr. Brian Staiger, PharmD, BCPS

A pharmacist-reviewed analysis of the Ethosuximide and Ritonavir 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

Ritonavir

Norvir

Ethosuximide

Zarontin Zarontin®

Medically reviewed by

Dr. Brian Staiger, PharmD, BCPS

Updated Aug 8, 2026

Editorial policy · Data sources

Interaction severity

Moderate

Can be significant — usually manageable with 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 14 documented Ethosuximide interactions, 6 are rated moderate — including this one.

Onset

delayed

Evidence

theoretical

Severity

Moderate

What happens

Increased ethosuximide serum concentrations and potential toxicity (nausea, drowsiness, dizziness, weakness, headache)

Interaction Deep Dive

Coadministered ritonavir may significantly increase serum concentrations of ethosuximide, and may increase ethosuximide toxicity. When coadministered with ritonavir, monitor ethosuximide therapeutic concentrations, if available, and reduce ethosuximide as required¹.

Why it happens (mechanism)

Decreased ethosuximide metabolism

Common questions

Can I take Ethosuximide and Ritonavir together?

Increased ethosuximide serum concentrations and potential toxicity (nausea, drowsiness, dizziness, weakness, headache) Always confirm with your pharmacist or prescriber before making any change.

How serious is the Ethosuximide and Ritonavir interaction?

It is rated moderate. Can be significant — usually manageable with 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 "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.

Questions for your pharmacist

Does my dose of Ethosuximide or Ritonavir 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 anything you'd monitor while I'm on both?

References (1)

Product Information: NORVIR(R), ritonavir capsules, ritonavir oral solution. Abbott Laboratories, Abbott Park, IL, 2005. DailyMed

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Ritonavir

Keep reading about Ritonavir

All Ritonavir interactions (174)

Also serious with: Midazolam Nasal Spray Alfuzosin Atorvastatin Simvastatin

Ethosuximide

Keep reading about Ethosuximide

All Ethosuximide interactions (14) Ethosuximide drug information

Also serious with: Valproic Acid Fosphenytoin Hydroxychloroquine Nevirapine

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:

Ritonavir + Activated Charcoal Beer Grapefruit Siberian Cocklebur St. John's Wort Wine 1,4-butanediol Acacia Rigidula + 170 more

Ethosuximide + 1,4-butanediol Activated Charcoal Beer Gamma-hydroxybutyrate (ghb) Grapefruit Kava L-tryptophan St. John's Wort + 218 more

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

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A message from HelloPharmacist

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.