Drug interactions
Fosphenytoin + Ritonavir
Severity: Major
Fosphenytoin and Ritonavir: 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
Ritonavir may lower phenytoin concentrations through enzyme induction, risking loss of seizure control. Phenytoin may also lower ritonavir exposure, which can reduce antiretroviral efficacy.
What this interaction means in plain words
Fosphenytoin 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.
Fosphenytoin belongs to Central Nervous System; Ritonavir belongs to Anti Virals.
Why it happens
Induction of CYP2C9 and CYP3A4 (and glucuronidation) lowers concentrations of both drugs, though ritonavir also inhibits some pathways, so the net effect is variable.
What to do
Monitor phenytoin levels and seizure control, and adjust the dose accordingly. Monitor HIV viral load and ritonavir exposure, or choose an alternative anticonvulsant.
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 Fosphenytoin
- Dronedarone + Fosphenytoin Contraindicated
- Fosphenytoin + Ledipasvir-sofosbuvir Contraindicated
- Fosphenytoin + Lorlatinib Contraindicated
- Fosphenytoin + Lurasidone Contraindicated
- Fosphenytoin + Ranolazine Contraindicated
- Fosphenytoin + Sofosbuvir-velpatasvir-voxilaprevir 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 Fosphenytoin with Ritonavir?
MediHelp rates the interaction as major. Fosphenytoin 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. Ritonavir may lower phenytoin concentrations through enzyme induction, risking loss of seizure control. Phenytoin may also lower ritonavir exposure, which can reduce antiretroviral efficacy.
How serious is the interaction between Fosphenytoin 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 levels and seizure control, and adjust the dose accordingly. Monitor HIV viral load and ritonavir exposure, or choose an alternative anticonvulsant.
What other medicines interact with Fosphenytoin?
MediHelp also lists interactions of Fosphenytoin with, for example, Dronedarone, Ledipasvir-sofosbuvir, Lorlatinib. The full list is on the Fosphenytoin page.
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