Simvastatin and Phenobarbital: 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
Phenobarbital
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.
Let's talk about taking phenobarbital and simvastatin (Zocor) together. Phenobarbital revs up a liver enzyme called CYP3A, and that enzyme is the one that breaks down simvastatin. So when both are on board, your body may clear the simvastatin faster than usual.
The practical worry is that your cholesterol medicine may not work as well, meaning your LDL ("bad" cholesterol) might not come down as much as your doctor wants. This is based mostly on how the drugs work in theory, not on a lot of hard proof, so please don't panic. The good news is this is very manageable. Keep taking both as prescribed and let your care team know you're on them, so they can check your cholesterol and adjust things if needed.
Effect: Phenobarbital is a potent inducer of CYP3A4/5, the primary enzyme metabolizing simvastatin. Concomitant use is expected to increase simvastatin (and its active acid metabolite) clearance, lowering AUC/Cmax and potentially reducing lipid-lowering efficacy.
- Direction: reduced simvastatin exposure and effect (simvastatin is not a prodrug requiring CYP3A activation; induction lowers active drug).
- Evidence: theoretical/mechanistic.
- Onset: unspecified; enzyme induction typically develops over 1 to 2 weeks and reverses similarly after stopping the inducer.
Management: Monitor lipid panel response. Simvastatin dose may need to be individualized and titrated upward, or an alternative statin considered, based on LDL response.
What happens
Reduced CYP3A substrate exposure and reduced efficacy of CYP3A substrate
Interaction Deep Dive
When PHENobarbital, which acts as a CYP3A inducer, is taken together with a substrate of CYP3A, the exposure to that CYP3A substrate can fall, potentially diminishing how well the substrate works. In situations where such combined use is unavoidable, keep a close watch on neonates 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
This interaction is very manageable, and you should keep taking both medicines as prescribed unless your prescriber tells you otherwise.
- Your care team may check your cholesterol (lipid panel) to see whether the simvastatin is still working well.
- Because phenobarbital can speed up how your body clears simvastatin, your simvastatin dose may need to be adjusted and individualized by your care team, or they may choose a different cholesterol medicine.
- Tell your pharmacist and doctor that you take both, and mention any new or stopped medicines.
Don't change either dose on your own. Your team can tailor treatment and monitor you to keep your cholesterol controlled.
Management is individual — confirm any change with your pharmacist or prescriber.
Common questions
Can I take Simvastatin and Phenobarbital together?
Phenobarbital can make simvastatin work less well by clearing it faster, so your team may monitor your cholesterol and adjust your statin dose or choose an alternative. Keep taking both as prescribed and check in with your pharmacist or doctor. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Simvastatin 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 "unspecified". The timing of this interaction is not well characterized.
How is the Simvastatin and Phenobarbital interaction managed?
This interaction is very manageable, and you should keep taking both medicines as prescribed unless your prescriber tells you otherwise. Your care team may check your cholesterol (lipid panel) to see whether the simvastatin is still working well. Because phenobarbital can speed up how your body clears simvastatin, your simvastatin dose may need to be adjusted and individualized by your care team,… 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 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 (1)
- Product Information: SEZABY(TM) intravenous injection, phenobarbital sodium intravenous injection. Sun Pharmaceutical Industries Inc (per manufacturer), Cranbury, NJ, 2022. DailyMed
Keep reading about Phenobarbital
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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