Anastrozole and Tamoxifen: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Jul 11, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Tamoxifen
Anastrozole
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Reduced anastrozole exposure
Interaction Deep Dive
Do not coadminister tamoxifen and anastrozole as it may reduce anastrozole exposure. In the anastrozole adjuvant trial, coadministration of anastrozole and tamoxifen citrate in breast cancer patients reduced anastrozole exposure by 27% compared to those achieved with anastrozole alone; however, the coadministration did not affect the pharmacokinetics of tamoxifen or N-desmethyltamoxifen1.
Why it happens (mechanism)
Unknown
Literature reports
1 report — tap to read
a) In the anastrozole adjuvant trial, coadministration of anastrozole and tamoxifen citrate in breast cancer patients reduced anastrozole exposure by 27% compared to those achieved with anastrozole alone; however, the coadministration did not affect the pharmacokinetics of tamoxifen or N-desmethyltamoxifen 1.
Common questions
Can I take Anastrozole and Tamoxifen together?
Reduced anastrozole exposure Always confirm with your pharmacist or prescriber before making any change.
How serious is the Anastrozole and Tamoxifen interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "unspecified". The timing of this interaction is not well characterized.
How strong is the evidence for this interaction?
The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Anastrozole or Tamoxifen need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there a safer alternative to one of these medications for me?
References (1)
- Product Information: Tamoxifen Citrate oral tablets, tamoxifen citrate oral tablets. Teva Pharmaceuticals (per DailyMed), Parsippany, NJ, 2025. DailyMed
Keep reading about Tamoxifen
Keep reading about Anastrozole
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
Check another combination
Our instant two-drug interaction checker is almost here.
Still have questions about this combination?
Every question gets a real answer from a licensed pharmacist — free, and usually within a day.
Ask the pharmacist