💊 Tamoxifen is a selective oestrogen receptor modulator (SERM) used mainly in the treatment and prevention of oestrogen receptor–positive (ER⁺) breast cancer.
It acts as an antagonist in breast tissue but an agonist in bone, uterus, and lipid metabolism.
By competitively binding to oestrogen receptors, it inhibits oestrogen-driven tumour growth.
The main long-term risk is venous thromboembolism (VTE) and, rarely, endometrial carcinoma.
📘 About
- Always check the BNF entry for current dosing and contraindication guidance.
- Commonly used for adjuvant therapy in ER⁺ breast cancer (pre- or postmenopausal) and for risk reduction in high-risk individuals.
- Also used off-label to induce ovulation in selected infertility cases.
⚙️ Mode of Action
- Competitive antagonist at oestrogen receptors in breast tissue - blocks oestrogen-driven DNA synthesis and cell proliferation.
- Acts as a partial agonist at oestrogen receptors in bone and endometrium - preserving bone density but potentially stimulating uterine lining.
- Overall effect: ↓ recurrence and mortality in ER⁺ breast cancer by reducing tumour cell growth.
💊 Indications & Dose
- Oestrogen receptor–positive breast cancer: Tamoxifen 20 mg PO once daily (usually long-term, up to 5–10 years).
- Fertility induction (off-label): 20 mg PO once daily for 5 days early in menstrual cycle; higher doses may be used under specialist supervision.
💉 Dose Table (Typical Ranges)
| Indication |
Starting Dose |
Frequency |
Route |
| Breast cancer (ER⁺) |
20 mg |
Once daily |
Oral |
| Fertility disorders (off-label) |
20 mg (higher doses may be required) |
Once daily for 5 days |
Oral |
🔄 Interactions
- Concurrent use with SSRIs such as paroxetine or fluoxetine may reduce tamoxifen efficacy (inhibits CYP2D6 → ↓ active metabolite endoxifen).
- Increased VTE risk with oestrogen-containing therapies or immobility.
- See BNF for full interaction list.
⚠️ Cautions
- History of VTE or thrombophilia - use with caution; ensure thromboprophylaxis if immobilised.
- Monitor for endometrial changes - report abnormal vaginal bleeding or discharge.
- May cause menstrual irregularity or ovarian cysts in premenopausal women.
🚫 Contraindications
- Pregnancy - teratogenic; ensure contraception during and for at least 2 months after treatment.
- Breastfeeding - avoid use.
- Concurrent coumarin anticoagulant use (relative contraindication due to bleeding risk).
💥 Adverse Effects
- 🔥 Hot flushes and night sweats (due to anti-oestrogenic effects).
- ⬆️ VTE risk (deep vein thrombosis, pulmonary embolism).
- 💧 Vaginal discharge, pruritus, or dryness.
- 🧬 Endometrial hyperplasia or carcinoma (rare; due to uterine agonist activity).
- GI disturbance, nausea, leg cramps, or fluid retention.
🧠 Teaching Note
Tamoxifen exemplifies a drug with tissue-selective receptor modulation - antagonistic in breast, agonistic in bone and endometrium.
It improves 10-year survival in ER⁺ breast cancer but requires vigilance for thrombotic and uterine complications.
When combined with SSRIs for menopausal symptoms, always select a non-CYP2D6–inhibiting SSRI such as citalopram or sertraline.
📚 References
- BNF: Tamoxifen
- NICE NG101: Early and locally advanced breast cancer (2023)
- EBCTCG (Lancet 2011): 15-year survival benefit from 5 years of adjuvant tamoxifen
🕓 Revisions
- 2025-10 - Expanded and annotated by Dr O’Kane (Makindo edition).
