Drug interactions
Fentanyl + Fosphenytoin
Severity: Major
Fentanyl and Fosphenytoin: major 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, a CYP3A4 inducer, which can lower fentanyl levels and cause loss of analgesia or opioid withdrawal. Stopping the inducer later can lead to a rise in fentanyl concentrations and toxicity.
What this interaction means in plain words
Fentanyl and Fosphenytoin 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.
Fentanyl belongs to Musculoskeletal; Fosphenytoin belongs to Central Nervous System.
Why it happens
Phenytoin induces CYP3A4, increasing fentanyl metabolic clearance.
What to do
Monitor analgesic response and withdrawal symptoms and increase the fentanyl dose as needed. If the anticonvulsant is withdrawn, lower the fentanyl dose and monitor for respiratory depression.
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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Other interactions of Fentanyl
- Alvimopan + Fentanyl Contraindicated
- Fentanyl + Isocarboxazid Contraindicated
- Fentanyl + Olanzapine / samidorphan Contraindicated
- Fentanyl + Phenelzine Contraindicated
- Fentanyl + Tranylcypromine Contraindicated
- Alfentanil + Fentanyl Major
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 Fentanyl with Fosphenytoin?
MediHelp rates the interaction as major. Fentanyl and Fosphenytoin 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 is converted to phenytoin, a CYP3A4 inducer, which can lower fentanyl levels and cause loss of analgesia or opioid withdrawal. Stopping the inducer later can lead to a rise in fentanyl concentrations and toxicity.
How serious is the interaction between Fentanyl and Fosphenytoin?
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 analgesic response and withdrawal symptoms and increase the fentanyl dose as needed. If the anticonvulsant is withdrawn, lower the fentanyl dose and monitor for respiratory depression.
What other medicines interact with Fentanyl?
MediHelp also lists interactions of Fentanyl with, for example, Alvimopan, Isocarboxazid, Olanzapine / samidorphan. The full list is on the Fentanyl page.
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