Drug interactions
Larotrectinib + Phenytoin
Severity: Major
Larotrectinib and Phenytoin: 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
Phenytoin can lower larotrectinib concentrations through CYP3A4 induction and may reduce its antitumor efficacy in TRK fusion cancers.
What this interaction means in plain words
Larotrectinib and Phenytoin 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.
Larotrectinib belongs to Oncology; Phenytoin belongs to Central Nervous System.
Why it happens
Phenytoin induces CYP3A4, increasing larotrectinib clearance.
What to do
Avoid if possible. If unavoidable, double the larotrectinib dose and return to the prior dose after the inducer has been stopped for about 3 to 5 half-lives.
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 Phenytoin
- Artemether / lumefantrine + Phenytoin Contraindicated
- Bictegravir + Phenytoin Contraindicated
- Cabotegravir + Phenytoin Contraindicated
- Doravirine + Phenytoin Contraindicated
- Elbasvir / grazoprevir + Phenytoin Contraindicated
- Elvitegravir / cobicistat / emtricitabine / tenofovir df + Phenytoin Contraindicated
Other interactions of Larotrectinib
Frequently asked questions
Can I take Larotrectinib with Phenytoin?
MediHelp rates the interaction as major. Larotrectinib and Phenytoin 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. Phenytoin can lower larotrectinib concentrations through CYP3A4 induction and may reduce its antitumor efficacy in TRK fusion cancers.
How serious is the interaction between Larotrectinib and Phenytoin?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Avoid if possible. If unavoidable, double the larotrectinib dose and return to the prior dose after the inducer has been stopped for about 3 to 5 half-lives.
What other medicines interact with Phenytoin?
MediHelp also lists interactions of Phenytoin with, for example, Artemether / lumefantrine, Bictegravir, Cabotegravir. The full list is on the Phenytoin page.
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