Drug interactions
Lapatinib + Tamoxifen
Severity: Minor
Lapatinib and Tamoxifen: 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
Lapatinib is a CYP3A4 inhibitor and may slow the conversion of tamoxifen to N-desmethyltamoxifen. Because this metabolite is also active and is further converted to endoxifen, the net effect on efficacy is probably small. Both drugs can also prolong the QT interval, adding a minor additive concern.
What this interaction means in plain words
Lapatinib and Tamoxifen have a small, usually unimportant interaction. Most people can take both; the pharmacist may still mention it so that any unexpected effect is recognised.
Lapatinib belongs to Oncology; Tamoxifen belongs to Endocrine.
Why it happens
Lapatinib inhibition of CYP3A4 reduces tamoxifen N-demethylation, plus possible additive QT effects.
What to do
No routine dose change is needed. Consider an ECG and electrolyte monitoring if other QT-prolonging risks are present.
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 Tamoxifen
- Amiodarone + Tamoxifen Major
- Anastrozole + Tamoxifen Major
- Bupropion + Tamoxifen Major
- Carbamazepine + Tamoxifen Major
- Cinacalcet + Tamoxifen Major
- Dacomitinib + Tamoxifen Major
Other interactions of Lapatinib
- Dofetilide + Lapatinib Contraindicated
- Ibutilide + Lapatinib Contraindicated
- Lapatinib + Pimozide Contraindicated
- Lapatinib + Saquinavir Contraindicated
- Lapatinib + Thioridazine Contraindicated
- Adagrasib + Lapatinib Major
Frequently asked questions
Can I take Lapatinib with Tamoxifen?
MediHelp rates the interaction as minor. Lapatinib and Tamoxifen have a small, usually unimportant interaction. Most people can take both; the pharmacist may still mention it so that any unexpected effect is recognised. Lapatinib is a CYP3A4 inhibitor and may slow the conversion of tamoxifen to N-desmethyltamoxifen. Because this metabolite is also active and is further converted to endoxifen, the net effect on efficacy is probably small. Both drugs can also prolong the QT interval, adding a minor additive concern.
How serious is the interaction between Lapatinib and Tamoxifen?
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 routine dose change is needed. Consider an ECG and electrolyte monitoring if other QT-prolonging risks are present.
What other medicines interact with Tamoxifen?
MediHelp also lists interactions of Tamoxifen with, for example, Amiodarone, Anastrozole, Bupropion. The full list is on the Tamoxifen page.
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