Drug interactions
Fosphenytoin + Ondansetron
Severity: Minor
Fosphenytoin and Ondansetron: minor 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
Phenytoin, the active metabolite of fosphenytoin, induces CYP3A4 and can accelerate ondansetron clearance. Studies with phenytoin and other inducers showed lower ondansetron exposure, though dose changes are generally not required.
What this interaction means in plain words
Fosphenytoin and Ondansetron have a small, usually unimportant interaction. Most people can take both; the pharmacist may still mention it so that any unexpected effect is recognised.
Fosphenytoin belongs to Central Nervous System; Ondansetron belongs to Gastrointestinal Tract.
Why it happens
Enzyme induction (CYP3A4) by phenytoin increases ondansetron metabolism.
What to do
No empirical dose change is advised. Monitor antiemetic response and consider a higher dose or alternative if nausea control is poor.
What to ask your pharmacist
- Is there anything I should notice while taking both?
- Do I need to tell my doctor at the next visit?
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Other interactions of Ondansetron
- Apomorphine + Ondansetron Contraindicated
- Ondansetron + Thioridazine Contraindicated
- Ondansetron + Vandetanib Contraindicated
- Adagrasib + Ondansetron Major
- Amiodarone + Ondansetron Major
- Amisulpride + Ondansetron 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 Fosphenytoin with Ondansetron?
MediHelp rates the interaction as minor. Fosphenytoin and Ondansetron have a small, usually unimportant interaction. Most people can take both; the pharmacist may still mention it so that any unexpected effect is recognised. Phenytoin, the active metabolite of fosphenytoin, induces CYP3A4 and can accelerate ondansetron clearance. Studies with phenytoin and other inducers showed lower ondansetron exposure, though dose changes are generally not required.
How serious is the interaction between Fosphenytoin and Ondansetron?
MediHelp rates it minor, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: No empirical dose change is advised. Monitor antiemetic response and consider a higher dose or alternative if nausea control is poor.
What other medicines interact with Ondansetron?
MediHelp also lists interactions of Ondansetron with, for example, Apomorphine, Thioridazine, Vandetanib. The full list is on the Ondansetron page.
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