Drug interactions
Fosphenytoin + Lamotrigine
Severity: Moderate
Fosphenytoin and Lamotrigine: 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 lowers lamotrigine plasma concentrations substantially. This can lead to loss of seizure control if the lamotrigine dose is not adjusted, and the effect reverses when phenytoin is stopped.
What this interaction means in plain words
Fosphenytoin and Lamotrigine 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; Lamotrigine belongs to Central Nervous System.
Why it happens
Phenytoin induces hepatic UDP-glucuronosyltransferase, increasing glucuronidation and clearance of lamotrigine.
What to do
Use the lamotrigine dosing schedule recommended for patients taking enzyme-inducing antiepileptics, and monitor seizure control. Reduce the lamotrigine dose if the inducer is withdrawn.
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 Lamotrigine
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 Lamotrigine?
MediHelp rates the interaction as moderate. Fosphenytoin and Lamotrigine 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 lowers lamotrigine plasma concentrations substantially. This can lead to loss of seizure control if the lamotrigine dose is not adjusted, and the effect reverses when phenytoin is stopped.
How serious is the interaction between Fosphenytoin and Lamotrigine?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Use the lamotrigine dosing schedule recommended for patients taking enzyme-inducing antiepileptics, and monitor seizure control. Reduce the lamotrigine dose if the inducer is withdrawn.
What other medicines interact with Lamotrigine?
MediHelp also lists interactions of Lamotrigine with, for example, Desogestrel-ethinyl estradiol, Divalproex, Dofetilide. The full list is on the Lamotrigine page.
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