Fosphenytoin and Simvastatin: 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
Fosphenytoin
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 fosphenytoin (a seizure medicine that turns into phenytoin in your body) together with simvastatin (a cholesterol medicine). Phenytoin speeds up the liver system that breaks down simvastatin. That means your body clears the simvastatin faster, so there's less of it around to lower your cholesterol.
The main worry here isn't a dangerous side effect. It's that your simvastatin may not work as well as it should. The good news is this is very manageable. Your care team can check your cholesterol and adjust your simvastatin dose so it keeps doing its job. Just keep taking both as prescribed and let your pharmacist or doctor know you're on both.
Effect: Reduced simvastatin exposure (decreased efficacy), a PK interaction.
Mechanism: Phenytoin (fosphenytoin is its prodrug) is a potent inducer of hepatic CYP3A4. Simvastatin is a CYP3A4 substrate, so induction increases its metabolism and lowers systemic exposure (AUC/Cmax).
- Direction: Reduced statin effect, potential loss of lipid control.
- Onset: Delayed (enzyme induction develops over days to weeks).
- Evidence: Probable; severity major.
Management: Monitor lipid panel. Simvastatin dose may need upward titration and individualization while on phenytoin/fosphenytoin, and re-evaluation if the inducer is withdrawn (risk of overshoot).
What happens
Reduced simvastatin exposure
Interaction Deep Dive
Taking phenytoin or fosphenytoin (the prodrug of phenytoin) together with simvastatin has led to decreased simvastatin exposure. When phenytoin or fosphenytoin is either started or stopped in a patient's regimen, consider adjusting the simvastatin dose to reach the desired clinical response123.
Why it happens (mechanism)
Induction of CYP3A4-mediated metabolism of simvastatin; induction of hepatic drug-metabolizing enzymes by phenytoin
How to manage this interaction
Here's how a care team typically handles this:
- Keep taking both medicines as prescribed unless your prescriber tells you otherwise.
- Your team may check your cholesterol (lipid panel) to see how well the simvastatin is working.
- Your simvastatin dose may be adjusted and individualized to get the best response while you're on fosphenytoin.
- If the fosphenytoin is ever stopped, your team may re-check and readjust your simvastatin, since the interaction fades and levels can rise again.
Ask your pharmacist or doctor to review both medicines together so they can monitor and tailor your dose.
Management is individual — confirm any change with your pharmacist or prescriber.
Literature reports
1 report — tap to read
a) A 50-year-old woman was taking simvastatin 10 mg daily for familial hypercholesterolemia, with a total cholesterol level of 9.4 mmol/L and a triglyceride level of 1.87 mmol/L. Her other medications included sodium valproate 200 mg three times daily for epilepsy. Her anticonvulsant was changed to phenytoin 325 mg daily, and three months afterward her cholesterol had risen to 15.99 mmol/L. Serum activity of gamma-glutamyl transpeptidase, which had been within the normal range (5 Units/L to 50 Units/L) prior to phenytoin, was also raised at 175 Units/L. Simvastatin was titrated up to 40 mg in a step-wise manner, without resolution. Her anti-lipidemic treatment was changed to fluvastatin 40 mg daily and ultimately to atorvastatin, beginning at 10 mg daily and increasing in a step-wise fashion to 80 mg daily. Her cholesterol remained above 10 mmol/L throughout these adjustments, while phenytoin concentrations stayed within the therapeutic range. Phenytoin was gradually withdrawn, and her total cholesterol fell to 6.24 mmol/L while gamma-glutamyl transpeptidase returned to normal values 4.
Common questions
Can I take Fosphenytoin and Simvastatin together?
Fosphenytoin speeds up the breakdown of simvastatin, which can make your cholesterol medicine less effective. Your care team can manage this by checking your cholesterol and adjusting your simvastatin dose, so keep taking both as prescribed and tell them you're on both. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Fosphenytoin and Simvastatin 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 is the Fosphenytoin and Simvastatin interaction managed?
Here's how a care team typically handles this: Keep taking both medicines as prescribed unless your prescriber tells you otherwise. Your team may check your cholesterol (lipid panel) to see how well the simvastatin is working. Your simvastatin dose may be adjusted and individualized to get the best response while you're on fosphenytoin. If the fosphenytoin is ever stopped, your team may re-check a… Management is individual — always follow your own care team's guidance.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Fosphenytoin or Simvastatin 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 (4)
- Product Information: FOSPHENYTOIN SODIUM intravenous and intramuscular injection, fosphenytoin sodium intravenous and intramuscular injection. SAGENT Pharmaceuticals (per Dailymed), Schaumburg, IL, 2024. DailyMed
- Product Information: PHENYTOIN SODIUM intravenous and intramuscular injection, phenytoin sodium intravenous and intramuscular injection. Acella Pharmaceuticals, LLC (per Dailymed), Alpharetta, GA, 2024. DailyMed
- Product Information: PHENYTOIN SODIUM oral extended-release capsules, phenytoin sodium oral extended-release capsules. Sun Pharmaceutical Industries, Inc. (per Dailymed), Cranbury, NJ, 2023. DailyMed
- Murphy MJ & Dominiczak MH: Efficacy of statin therapy: possible effect of phenytoin. Postgrad Med 1999; 75:359-360. PubMed
Keep reading about Fosphenytoin
Keep reading about Simvastatin
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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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