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# Tamoxifen

A breast cancer tablet that blocks oestrogen in the breast but acts like it in the uterus and bone. It raises the risk of clots and uterine cancer.

Updated 2026-11-01

- Class: [Selective oestrogen receptor modulators (SERMs)](https://durgical.com/en/pharmacology/classes/selective-oestrogen-receptor-modulators/)
- Receptor: Oestrogen receptor alpha (ERα)
- ATC code: L02BA01
- WHO Model List of Essential Medicines

◎**Oestrogen receptor alpha (ERα)**Tamoxifen

ⓘ The figure shows 4-hydroxytamoxifen, an active metabolite of tamoxifen, in the ligand-binding domain of human ERα; its side chain keeps helix 12 out of the active position.

**Target in 3D:** Oestrogen receptor alpha (ERα). PDB structure 3ERT (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/3ERT) · Drag to rotate · pinch to zoom

## Mechanism of action

Tamoxifen and its active metabolites bind the oestrogen receptor and compete with estradiol. In breast tissue the complex fails to recruit co-activators, so oestrogen-driven genes stay off and receptor-positive tumour cells stop proliferating. In the endometrium, bone and liver it behaves as a partial agonist, which explains endometrial thickening, preserved bone after menopause and changes in clotting factors.

- Liver enzymes form active metabolites
- They bind the oestrogen receptor instead of estradiol
- In the breast the receptor stays inactive: tumour growth stops
- In uterus and bone it acts partly like oestrogen

## Pharmacokinetics

**Absorption**
Taken by mouth

**Metabolism**
Liver, by CYP3A, CYP2C9 and CYP2D6, to active metabolites such as N-desmethyltamoxifen

**Elimination**
Mainly in the faeces

**Half-life**
About 5 to 7 days; its main metabolite about 2 weeks

**Onset and duration**
Steady state takes weeks

## Indications

- Hormone receptor-positive breast cancer, adjuvant or metastatic, in women and men [Breast cancer →](https://durgical.com/en/conditions/breast-cancer/)
- Ductal carcinoma in situ after surgery and radiation, to lower the risk of invasive cancer [Breast cancer →](https://durgical.com/en/conditions/breast-cancer/)
- Reduction of breast cancer risk in women at high risk [Breast cancer →](https://durgical.com/en/conditions/breast-cancer/)

## Adverse effects

### Common

- Hot flashes
- Vaginal discharge or bleeding
- Menstrual changes in premenopausal women
- Nausea
- Fluid retention

### Serious (rare, important)

- Endometrial cancer and uterine sarcoma
- Deep vein thrombosis and pulmonary embolism
- Stroke
- Cataracts
- Hypercalcaemia in patients with bone metastases

## Contraindications

- Hypersensitivity to tamoxifen
- For risk reduction or ductal carcinoma in situ: history of deep vein thrombosis or pulmonary embolism, or need for coumarin anticoagulants

## Key interactions

**Warfarin and other coumarin anticoagulants**
Much stronger anticoagulant effect; prothrombin time is monitored closely.

**Letrozole**
Tamoxifen lowers letrozole levels.

**Anastrozole**
The combination adds no benefit and is not used.

**Cytotoxic chemotherapy**
Higher risk of thromboembolic events.

## What is monitored

- Gynaecological checks and any abnormal vaginal bleeding
- Signs of thrombosis: leg swelling, chest pain, shortness of breath
- Eye examination for cataracts
- Blood counts and liver tests periodically
- Calcium in patients with bone metastases

## Administration and safety

- Oral tablet or solution, at the same time each day.
- Hazardous drug: handle broken tablets with care, especially during pregnancy.

## Nursing care

- Teach the patient to report any abnormal vaginal bleeding, pelvic pain or pressure without delay.
- Teach the signs of a clot and stroke: one swollen or painful leg, chest pain, sudden breathlessness, sudden weakness or speech trouble.
- Ask about warfarin and other anticoagulants before starting.
- Hot flashes are common; report them if they are severe.
- Non-hormonal contraception is needed in premenopausal women.

## Pregnancy and breastfeeding

Can cause fetal harm; pregnancy is avoided during treatment and for about two months after, using non-hormonal contraception.

## Memory aid

TAMoxifen TAMes the breast but wakes the uterus: block in the breast, stimulate the endometrium, clot the veins.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does tamoxifen raise the risk of endometrial cancer?

It is a partial oestrogen agonist in the endometrium, so it stimulates endometrial growth even while it blocks oestrogen in the breast.

## What happens to the INR when tamoxifen is added to warfarin?

The anticoagulant effect increases significantly, so the INR can rise and bleeding becomes more likely; prothrombin time is monitored closely.

## Why can tamoxifen be used in premenopausal women while aromatase inhibitors alone cannot?

Tamoxifen blocks the oestrogen receptor whatever the source of oestrogen, including the ovary; aromatase inhibitors alone cannot stop ovarian oestrogen production.

## A woman on tamoxifen has sudden pain and swelling in one calf. What should be suspected?

Deep vein thrombosis, a known serious adverse effect of tamoxifen; it needs urgent assessment.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/pharmacology/tamoxifen/)

## Related drugs

- [Raloxifene](https://durgical.com/en/pharmacology/raloxifene/)
- [Anastrozole](https://durgical.com/en/pharmacology/anastrozole/)
- [Letrozole](https://durgical.com/en/pharmacology/letrozole/)

## Related conditions

- [Breast cancer](https://durgical.com/en/conditions/breast-cancer/)

[← Back to the class: Selective oestrogen receptor modulators (SERMs)](https://durgical.com/en/pharmacology/classes/selective-oestrogen-receptor-modulators/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Tamoxifen: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a682414.html) (MedlinePlus (U.S. National Library of Medicine), 2018); [DailyMed - TAMOXIFEN CITRATE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=8c0eb763-420a-4106-ba56-526b53a7f72b) (DailyMed (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=L02BA01) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); [HUMAN ESTROGEN RECEPTOR ALPHA LIGAND-BINDING DOMAIN IN COMPLEX WITH 4-HYDROXYTAMOXIFEN](https://www.rcsb.org/structure/3ERT) (RCSB Protein Data Bank, 1999).

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Representative model, for educational purposes only.

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