Drug interactions
Fluorouracil + Fosphenytoin
Severity: Moderate
Fluorouracil and Fosphenytoin: moderate 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
Fluorouracil can increase serum phenytoin concentrations, and fosphenytoin is converted to phenytoin. Raised levels may cause nystagmus, ataxia, drowsiness and confusion, and loss of seizure control can occur if the dose is altered inappropriately.
What this interaction means in plain words
Fluorouracil and Fosphenytoin can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken.
Fluorouracil belongs to Oncology; Fosphenytoin belongs to Central Nervous System.
Why it happens
Probably reduced phenytoin clearance, possibly via CYP2C9 inhibition; the mechanism is not firmly established.
What to do
Monitor serum phenytoin levels and for signs of toxicity during fluorouracil therapy. Adjust the fosphenytoin dose according to levels and clinical response.
What to ask your pharmacist
- Should I take them at different times of day?
- Does the dose of either medicine need to change?
- Is there anything I should avoid, such as alcohol or certain foods, while on both?
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Other interactions of Fluorouracil
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 Fluorouracil with Fosphenytoin?
MediHelp rates the interaction as moderate. Fluorouracil and Fosphenytoin can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken. Fluorouracil can increase serum phenytoin concentrations, and fosphenytoin is converted to phenytoin. Raised levels may cause nystagmus, ataxia, drowsiness and confusion, and loss of seizure control can occur if the dose is altered inappropriately.
How serious is the interaction between Fluorouracil and Fosphenytoin?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor serum phenytoin levels and for signs of toxicity during fluorouracil therapy. Adjust the fosphenytoin dose according to levels and clinical response.
What other medicines interact with Fluorouracil?
MediHelp also lists interactions of Fluorouracil with, for example, Adenovirus types 4 and 7 live, Axicabtagene ciloleucel, Belatacept. The full list is on the Fluorouracil page.
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