Drug interactions
Fosphenytoin + Pyrimethamine
Severity: Moderate
Fosphenytoin and Pyrimethamine: 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
Pyrimethamine is an antifolate and phenytoin (the active form of fosphenytoin) can also lower folate status. Together they may add to folate deficiency and haematological toxicity. Reports of reduced anticonvulsant effect are inconsistent.
What this interaction means in plain words
Fosphenytoin and Pyrimethamine 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.
Fosphenytoin belongs to Central Nervous System; Pyrimethamine belongs to Anti Parasitic.
Why it happens
Additive folate antagonism; pyrimethamine inhibits dihydrofolate reductase and phenytoin impairs folate handling, and phenytoin levels may also change through unclear mechanisms.
What to do
Monitor complete blood counts and seizure control, and check phenytoin concentrations if control changes. Give folinic acid with pyrimethamine as usually recommended.
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?
Pro+ members
Unlock the full page
Create an account and start a Pro+ trial to unlock the full management advice for Fosphenytoin and Pyrimethamine, the additional notes, the evidence detail and the whole-prescription interaction checker.
- Full clinical management advice
- Additional notes
- Evidence detail
- Interaction checker for a whole prescription
Try Pro+ free for 14 days on the web. No card needed.
Other interactions of Pyrimethamine
- Deferiprone + Pyrimethamine Major
- Eliglustat + Pyrimethamine Major
- Acalabrutinib + Pyrimethamine Moderate
- Brexpiprazole + Pyrimethamine Moderate
- Dapsone + Pyrimethamine Moderate
- Folic acid + Pyrimethamine Moderate
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 Fosphenytoin with Pyrimethamine?
MediHelp rates the interaction as moderate. Fosphenytoin and Pyrimethamine 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. Pyrimethamine is an antifolate and phenytoin (the active form of fosphenytoin) can also lower folate status. Together they may add to folate deficiency and haematological toxicity. Reports of reduced anticonvulsant effect are inconsistent.
How serious is the interaction between Fosphenytoin and Pyrimethamine?
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 complete blood counts and seizure control, and check phenytoin concentrations if control changes. Give folinic acid with pyrimethamine as usually recommended.
What other medicines interact with Pyrimethamine?
MediHelp also lists interactions of Pyrimethamine with, for example, Deferiprone, Eliglustat, Acalabrutinib. The full list is on the Pyrimethamine page.
Full monographs, dosing and an interaction checker in the MediHelp app
Open the complete reference, check multi-drug interactions and use dosing tools in the MediHelp app.
