Drug interactions
Efavirenz + Tamoxifen
Severity: Moderate
Efavirenz and Tamoxifen: 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
Efavirenz is a CYP3A4 inducer, which can alter tamoxifen conversion to its metabolites, and its effect on endoxifen formation is uncertain. Possible reduced tamoxifen efficacy is a concern in breast cancer treatment.
What this interaction means in plain words
Efavirenz and Tamoxifen 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.
Efavirenz belongs to Anti Virals; Tamoxifen belongs to Endocrine.
Why it happens
Efavirenz modulates CYP3A4 and CYP2B6/2C9 activity involved in tamoxifen metabolism, with uncertain net effect on active metabolites.
What to do
Consider an antiretroviral not affecting CYP enzymes, such as an integrase inhibitor, if tamoxifen efficacy is critical. Otherwise monitor oncologic response closely.
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 Efavirenz
- Cobimetinib + Efavirenz Contraindicated
- Dienogest / estradiol valerate + Efavirenz Contraindicated
- Dihydroergotamine intranasal + Efavirenz Contraindicated
- Dihydroergotamine + Efavirenz Contraindicated
- Doravirine + Efavirenz Contraindicated
- Efavirenz + Elbasvir / grazoprevir Contraindicated
Other interactions of Tamoxifen
- Amiodarone + Tamoxifen Major
- Anastrozole + Tamoxifen Major
- Bupropion + Tamoxifen Major
- Carbamazepine + Tamoxifen Major
- Cinacalcet + Tamoxifen Major
- Dacomitinib + Tamoxifen Major
Frequently asked questions
Can I take Efavirenz with Tamoxifen?
MediHelp rates the interaction as moderate. Efavirenz and Tamoxifen 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. Efavirenz is a CYP3A4 inducer, which can alter tamoxifen conversion to its metabolites, and its effect on endoxifen formation is uncertain. Possible reduced tamoxifen efficacy is a concern in breast cancer treatment.
How serious is the interaction between Efavirenz and Tamoxifen?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Consider an antiretroviral not affecting CYP enzymes, such as an integrase inhibitor, if tamoxifen efficacy is critical. Otherwise monitor oncologic response closely.
What other medicines interact with Efavirenz?
MediHelp also lists interactions of Efavirenz with, for example, Cobimetinib, Dienogest / estradiol valerate, Dihydroergotamine intranasal. The full list is on the Efavirenz page.
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