Simvastatin and Primidone: Interaction Details
AI-assisted, pharmacist-reviewed · AI content regenerated Jul 11, 2026 · Source data updated Jul 4, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Primidone
Simvastatin
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.
Here's what's going on. Primidone (Mysoline) can speed up an enzyme in your liver called CYP3A. Simvastatin (Zocor), your cholesterol medicine, is broken down by that same enzyme. So when you take them together, your body may clear the simvastatin faster than usual, which can leave less of it working.
The practical worry is that your cholesterol medicine may not lower your cholesterol as well as expected. The good news is this is very manageable. Your care team can check your cholesterol numbers and adjust your simvastatin dose if needed. Keep taking both exactly as prescribed, and let your pharmacist or doctor know you're on both.
Mechanism: Primidone (and its metabolite phenobarbital) is a CYP3A inducer. Simvastatin is a CYP3A4 substrate (active drug, not a prodrug requiring 3A for activation).
- Direction: Enzyme induction increases simvastatin metabolism, lowering its plasma exposure (AUC/Cmax) and potentially reducing lipid-lowering efficacy.
- Magnitude: Not quantified; evidence is theoretical/extrapolated from CYP3A-inducer data.
- Onset: Unspecified; enzyme induction is typically delayed (days to weeks).
- Management: Monitor lipid panel; the simvastatin dose may need to be increased and individualized. Reassess after any change in primidone therapy.
What happens
Reduced CYP3A substrate exposure and reduced efficacy of CYP3A substrate
Interaction Deep Dive
When PHENobarbital, which induces CYP3A, is taken together with a substrate of CYP3A, the exposure to that substrate can fall, potentially lowering how well the CYP3A substrate works. Where such combined use is unavoidable, monitor neonates carefully and think about raising the CYP3A substrate dose as appropriate1.
Why it happens (mechanism)
Induction of CYP3A-mediated metabolism of CYP3A substrates
How to manage this interaction
Keep taking both medications as prescribed unless your prescriber tells you otherwise. This interaction can be managed.
- Your care team may check your cholesterol (lipid panel) to see whether the simvastatin is still working well enough.
- Your simvastatin dose may need to be adjusted and individualized based on those results.
- Tell your pharmacist and doctor that you take both drugs, and mention any recent start, stop, or dose change of primidone, since that can affect this.
Management is individual — confirm any change with your pharmacist or prescriber.
Common questions
Can I take Simvastatin and Primidone together?
Primidone can speed up how your body clears simvastatin, so your cholesterol medicine may work less well; your care team can monitor your cholesterol and adjust the simvastatin dose as needed. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Simvastatin and Primidone 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 is the Simvastatin and Primidone interaction managed?
Keep taking both medications as prescribed unless your prescriber tells you otherwise. This interaction can be managed. Your care team may check your cholesterol (lipid panel) to see whether the simvastatin is still working well enough. Your simvastatin dose may need to be adjusted and individualized based on those results. Tell your pharmacist and doctor that you take both drugs, and mention any… Management is individual — always follow your own care team's guidance.
How strong is the evidence for this interaction?
The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Simvastatin or Primidone 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 (1)
- Product Information: SEZABY(TM) intravenous injection, phenobarbital sodium intravenous injection. Sun Pharmaceutical Industries Inc (per manufacturer), Cranbury, NJ, 2022. DailyMed
Keep reading about Primidone
Keep reading about Simvastatin
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:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
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