Drug interactions
Phenytoin + Quinidine
Severity: Major
Phenytoin and Quinidine: major interaction.
Last updated
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 induces hepatic CYP3A4 and markedly lowers quinidine concentrations, which can lead to loss of antiarrhythmic effect. Stopping phenytoin later can cause quinidine levels to rise again, so toxicity is possible. Quinidine may also affect phenytoin levels.
What this interaction means in plain words
Phenytoin 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.
Phenytoin belongs to Central Nervous System; Quinidine belongs to Anti Parasitic.
Why it happens
Phenytoin induces CYP3A4 and increases quinidine clearance.
What to do
Monitor quinidine serum levels and ECG, and increase the quinidine dose as needed. Reduce the dose again if phenytoin is withdrawn, and consider an alternative antiarrhythmic.
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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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 Phenytoin
- Artemether / lumefantrine + Phenytoin Contraindicated
- Bictegravir + Phenytoin Contraindicated
- Cabotegravir + Phenytoin Contraindicated
- Doravirine + Phenytoin Contraindicated
- Elbasvir / grazoprevir + Phenytoin Contraindicated
- Elvitegravir / cobicistat / emtricitabine / tenofovir df + Phenytoin Contraindicated
Frequently asked questions
Can I take Phenytoin with Quinidine?
MediHelp rates the interaction as major. Phenytoin 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. Phenytoin induces hepatic CYP3A4 and markedly lowers quinidine concentrations, which can lead to loss of antiarrhythmic effect. Stopping phenytoin later can cause quinidine levels to rise again, so toxicity is possible. Quinidine may also affect phenytoin levels.
How serious is the interaction between Phenytoin 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 serum levels and ECG, and increase the quinidine dose as needed. Reduce the dose again if phenytoin is withdrawn, and consider an alternative antiarrhythmic.
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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