Drug interactions
Diltiazem + Fosphenytoin
Severity: Major
Diltiazem and Fosphenytoin: 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
Phenytoin, the active form of fosphenytoin, induces CYP3A4 and can substantially lower diltiazem levels, causing loss of effect. Diltiazem may in turn raise phenytoin concentrations and cause toxicity. Changes in either drug's dose or discontinuation can shift the balance.
What this interaction means in plain words
Diltiazem 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.
Diltiazem belongs to Cardiovascular; Fosphenytoin belongs to Central Nervous System.
Why it happens
Phenytoin induces CYP3A4 metabolism of diltiazem, while diltiazem inhibits CYP3A4 and may reduce phenytoin clearance.
What to do
Monitor phenytoin concentrations and signs of toxicity such as nystagmus and ataxia when diltiazem is started or changed. Watch for reduced diltiazem effect and adjust its dose or choose an alternative.
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 Diltiazem
- Diltiazem + Flibanserin Contraindicated
- Diltiazem + Ivabradine Contraindicated
- Diltiazem + Lomitapide Contraindicated
- Diltiazem + Lonafarnib Contraindicated
- Acebutolol + Diltiazem Major
- Alfentanil + Diltiazem 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 Diltiazem with Fosphenytoin?
MediHelp rates the interaction as major. Diltiazem 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, the active form of fosphenytoin, induces CYP3A4 and can substantially lower diltiazem levels, causing loss of effect. Diltiazem may in turn raise phenytoin concentrations and cause toxicity. Changes in either drug's dose or discontinuation can shift the balance.
How serious is the interaction between Diltiazem 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 phenytoin concentrations and signs of toxicity such as nystagmus and ataxia when diltiazem is started or changed. Watch for reduced diltiazem effect and adjust its dose or choose an alternative.
What other medicines interact with Diltiazem?
MediHelp also lists interactions of Diltiazem with, for example, Flibanserin, Ivabradine, Lomitapide. The full list is on the Diltiazem page.
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