Drug interactions
Fosphenytoin + Valproic acid
Severity: Major
Fosphenytoin and Valproic acid: major interaction.
Last updated Sources: US FDA label
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Valproate displaces phenytoin from albumin and inhibits its metabolism, which can raise free phenytoin concentrations and cause toxicity. Phenytoin products given with valproate have also been linked to hyperammonemia, and phenytoin can lower valproate levels by induction.
What this interaction means in plain words
Fosphenytoin and Valproic acid can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for.
Fosphenytoin belongs to Central Nervous System; Valproic acid belongs to Central Nervous System.
Why it happens
Protein-binding displacement and metabolic inhibition of phenytoin by valproate, plus phenytoin enzyme induction of valproate clearance.
What to do
Monitor free phenytoin concentrations rather than total levels, along with valproate levels, and adjust doses accordingly. Check ammonia if confusion, vomiting or lethargy develops.
What to ask your pharmacist
- Does the benefit of taking both outweigh the risk in my case?
- Do I need any blood test, ECG or other check while on both?
- What early warning signs should I watch for, and what should I do if they appear?
Pro+ members
Unlock the full page
Create an account and start a Pro+ trial to unlock the full management advice for Fosphenytoin and Valproic acid, the additional notes, the evidence detail and the whole-prescription interaction checker.
- Full clinical management advice
- Additional notes
- Evidence detail
- Interaction checker for a whole prescription
Try Pro+ free for 7 days on the web. No card needed.
References
Other interactions of Valproic acid
- Ertapenem + Valproic acid Contraindicated
- Imipenem / cilastatin / relebactam + Valproic acid Contraindicated
- Imipenem / cilastatin + Valproic acid Contraindicated
- Meropenem / vaborbactam + Valproic acid Contraindicated
- Meropenem + Valproic acid Contraindicated
- Calcium / magnesium / potassium / sodium oxybates + Valproic acid Major
Other interactions of Fosphenytoin
- Dronedarone + Fosphenytoin Contraindicated
- Fosphenytoin + Ledipasvir-sofosbuvir Contraindicated
- Fosphenytoin + Lorlatinib Contraindicated
- Fosphenytoin + Lurasidone Contraindicated
- Fosphenytoin + Ranolazine Contraindicated
- Fosphenytoin + Sofosbuvir-velpatasvir-voxilaprevir Contraindicated
Frequently asked questions
Can I take Fosphenytoin with Valproic acid?
MediHelp rates the interaction as major. Fosphenytoin and Valproic acid can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for. Valproate displaces phenytoin from albumin and inhibits its metabolism, which can raise free phenytoin concentrations and cause toxicity. Phenytoin products given with valproate have also been linked to hyperammonemia, and phenytoin can lower valproate levels by induction.
How serious is the interaction between Fosphenytoin and Valproic acid?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor free phenytoin concentrations rather than total levels, along with valproate levels, and adjust doses accordingly. Check ammonia if confusion, vomiting or lethargy develops.
What other medicines interact with Valproic acid?
MediHelp also lists interactions of Valproic acid with, for example, Ertapenem, Imipenem / cilastatin / relebactam, Imipenem / cilastatin. The full list is on the Valproic acid page.
Full monographs, dosing and an interaction checker in the MediHelp app
Open the complete reference, check multi-drug interactions and use dosing tools in the MediHelp app.
