Drug interactions
Diazepam + Fosphenytoin
Severity: Moderate
Diazepam 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
Fosphenytoin is converted to phenytoin, which can induce diazepam metabolism and lower its levels. Diazepam may in turn alter phenytoin levels unpredictably, and both agents together increase CNS depression, particularly with intravenous loading.
What this interaction means in plain words
Diazepam 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.
Diazepam belongs to Musculoskeletal; Fosphenytoin belongs to Central Nervous System.
Why it happens
Phenytoin induces CYP3A4 and CYP2C19, increasing diazepam clearance; additive CNS depression also occurs.
What to do
Monitor for reduced diazepam effect and for sedation or respiratory depression. Check phenytoin levels and adjust doses as needed.
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 Diazepam
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 Diazepam with Fosphenytoin?
MediHelp rates the interaction as moderate. Diazepam 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. Fosphenytoin is converted to phenytoin, which can induce diazepam metabolism and lower its levels. Diazepam may in turn alter phenytoin levels unpredictably, and both agents together increase CNS depression, particularly with intravenous loading.
How serious is the interaction between Diazepam 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 for reduced diazepam effect and for sedation or respiratory depression. Check phenytoin levels and adjust doses as needed.
What other medicines interact with Diazepam?
MediHelp also lists interactions of Diazepam with, for example, Calcium / magnesium / potassium / sodium oxybates, Alfentanil, Alprazolam. The full list is on the Diazepam page.
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