Drug interactions
Divalproex + Fosphenytoin
Severity: Major
Divalproex and Fosphenytoin: major interaction.
Last updated Sources: Health Canada product monograph
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
Phenytoin (from fosphenytoin) and valproate interact in both directions: valproate displaces phenytoin from albumin and inhibits its metabolism, so free phenytoin can rise while total levels look normal or low. Phenytoin induces valproate metabolism and lowers valproate levels, and the pair increases the risk of hyperammonemia and hepatotoxicity.
What this interaction means in plain words
Divalproex and Fosphenytoin 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.
Why it happens
Protein-binding displacement and metabolic inhibition of phenytoin by valproate, and enzyme induction of valproate metabolism by phenytoin.
What to do
Monitor free phenytoin levels and valproate levels, and watch for signs of phenytoin toxicity or loss of seizure control. Check ammonia if lethargy, vomiting or altered mental status occurs, and monitor liver function.
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?
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References
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 Divalproex with Fosphenytoin?
MediHelp rates the interaction as major. Divalproex and Fosphenytoin 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. Phenytoin (from fosphenytoin) and valproate interact in both directions: valproate displaces phenytoin from albumin and inhibits its metabolism, so free phenytoin can rise while total levels look normal or low. Phenytoin induces valproate metabolism and lowers valproate levels, and the pair increases the risk of hyperammonemia and hepatotoxicity.
How serious is the interaction between Divalproex and Fosphenytoin?
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 levels and valproate levels, and watch for signs of phenytoin toxicity or loss of seizure control. Check ammonia if lethargy, vomiting or altered mental status occurs, and monitor liver function.
What other medicines interact with Fosphenytoin?
MediHelp also lists interactions of Fosphenytoin with, for example, Dronedarone, Ledipasvir-sofosbuvir, Lorlatinib. The full list is on the Fosphenytoin page.
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