Drug interactions
Fosamprenavir + Phenytoin
Severity: Major
Fosamprenavir and Phenytoin: 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 may lower amprenavir levels when fosamprenavir is used unboosted, risking virologic failure. With ritonavir-boosted fosamprenavir, amprenavir levels may rise and phenytoin levels may fall.
What this interaction means in plain words
Fosamprenavir and Phenytoin 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
Phenytoin induces CYP3A4, while ritonavir boosting alters the net effect and may also affect phenytoin metabolism.
What to do
Monitor phenytoin concentrations and adjust the dose, and monitor HIV viral load. Consider an alternative anticonvulsant if virologic control is a concern.
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 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 Fosamprenavir with Phenytoin?
MediHelp rates the interaction as major. Fosamprenavir and Phenytoin 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 may lower amprenavir levels when fosamprenavir is used unboosted, risking virologic failure. With ritonavir-boosted fosamprenavir, amprenavir levels may rise and phenytoin levels may fall.
How serious is the interaction between Fosamprenavir and Phenytoin?
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 adjust the dose, and monitor HIV viral load. Consider an alternative anticonvulsant if virologic control is a concern.
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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