Drug interactions
Fosphenytoin + Quinidine
Severity: Major
Fosphenytoin and Quinidine: 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
Fosphenytoin is converted to phenytoin, a strong CYP3A4 inducer that substantially increases quinidine clearance. Quinidine concentrations can fall below the therapeutic range, leading to loss of antiarrhythmic effect.
What this interaction means in plain words
Fosphenytoin and Quinidine 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; Quinidine belongs to Anti Parasitic.
Why it happens
Phenytoin induces CYP3A4 and increases hepatic metabolism of quinidine.
What to do
Monitor quinidine concentrations and ECG and increase the quinidine dose as needed, or choose another antiarrhythmic. Reassess the dose if fosphenytoin is discontinued.
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 Quinidine
- Adagrasib + Quinidine Contraindicated
- Arsenic trioxide + Quinidine Contraindicated
- Artemether / lumefantrine + Quinidine Contraindicated
- Chloroquine + Quinidine Contraindicated
- Dofetilide + Quinidine Contraindicated
- Dronedarone + Quinidine Contraindicated
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 Quinidine?
MediHelp rates the interaction as major. Fosphenytoin and Quinidine 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 is converted to phenytoin, a strong CYP3A4 inducer that substantially increases quinidine clearance. Quinidine concentrations can fall below the therapeutic range, leading to loss of antiarrhythmic effect.
How serious is the interaction between Fosphenytoin and Quinidine?
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 quinidine concentrations and ECG and increase the quinidine dose as needed, or choose another antiarrhythmic. Reassess the dose if fosphenytoin is discontinued.
What other medicines interact with Quinidine?
MediHelp also lists interactions of Quinidine with, for example, Adagrasib, Arsenic trioxide, Artemether / lumefantrine. The full list is on the Quinidine page.
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