Drug interactions
Phenytoin + Ritonavir
Severity: Major
Phenytoin and Ritonavir: 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 can lower ritonavir exposure, which may reduce antiviral effect and its pharmacokinetic boosting of other protease inhibitors. Ritonavir can also alter phenytoin concentrations unpredictably, so seizure control or toxicity may change.
What this interaction means in plain words
Phenytoin and Ritonavir 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; Ritonavir belongs to Anti Virals.
Why it happens
Phenytoin induces CYP3A4 and P-glycoprotein, increasing ritonavir clearance, while ritonavir's mixed inhibition and induction of metabolic enzymes can change phenytoin levels.
What to do
Monitor phenytoin concentrations and HIV viral load closely, and consider a non-inducing antiseizure drug if feasible. Adjust phenytoin dose according to levels.
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 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
Other interactions of Ritonavir
- Alfuzosin + Ritonavir Contraindicated
- Amiodarone + Ritonavir Contraindicated
- Avanafil + Ritonavir Contraindicated
- Colchicine + Ritonavir Contraindicated
- Dronedarone + Ritonavir Contraindicated
- Eletriptan + Ritonavir Contraindicated
Frequently asked questions
Can I take Phenytoin with Ritonavir?
MediHelp rates the interaction as major. Phenytoin and Ritonavir 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 can lower ritonavir exposure, which may reduce antiviral effect and its pharmacokinetic boosting of other protease inhibitors. Ritonavir can also alter phenytoin concentrations unpredictably, so seizure control or toxicity may change.
How serious is the interaction between Phenytoin and Ritonavir?
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 HIV viral load closely, and consider a non-inducing antiseizure drug if feasible. Adjust phenytoin dose according to levels.
What other medicines interact with Phenytoin?
MediHelp also lists interactions of Phenytoin with, for example, Artemether / lumefantrine, Bictegravir, Cabotegravir. The full list is on the Phenytoin page.
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