Selective oestrogen receptor modulator (SERM) Pregnancy: Must not be administered during pregnancy. Women of childbearing potential should use barrier or other non-hormonal contraception and avoid pregnancy during treatment and for two months after cessation. Not recommended during breast-feeding.

Tamoxifen

Brand names: Soltamox, Nolvadex

Tamoxifen is a selective oestrogen-receptor modulator (SERM) used in oestrogen-receptor-positive breast cancer, for treatment and for risk reduction.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 20 mg

Route: Oral

Frequency: Once daily

Breast cancer (adults): recommended daily dose normally 20 mg; no additional benefit in recurrence or survival demonstrated with higher doses (30–40 mg/day has been used in some patients with advanced disease). Primary prevention of breast cancer in moderate/high-risk women: 20 mg daily for 5 years (specialist/shared-care initiation). Anovulatory infertility: 20 mg daily on days 2–5 of the menstrual cycle, with subsequent courses increased to 40 mg then 80 mg daily if ovulation not achieved. Not recommended in children.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

US labelling (FDA)

Reference — US labelling, may differ from UK

Metastatic breast cancer: 20-40 mg per day. For doses greater than 20 mg per day, administer in divided doses (morning and evening). ( 2 ) Adjuvant treatment of breast cancer, DCIS, reduction of breast cancer incidence in women at high risk: 20 mg per day ( 2 ) Metastatic Breast Cancer For patients with breast cancer, the recommended daily dose of SOLTAMOX is 20 to 40 mg. Doses greater than 20 mg per day should be given in divided doses (morning and evening). Adjuvant Treatment of Breast Cancer For use in the adjuvant setting, the recommended dose of SOLTAMOX is 20 mg daily for 5-10 years [see Clinical Studies (14.2) ] . Doses greater than 20 mg daily yield no additional clinical benefit. …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2021-11-29. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Contraindications

Pregnancy
Hypersensitivity to the active substance or to any of the excipients
Concurrent anastrozole therapy
For infertility treatment: personal or family history of confirmed idiopathic venous thromboembolism or a known genetic defect
For primary prevention: history of deep vein thrombosis or pulmonary embolism, or requirement for concomitant coumarin-type anticoagulant therapy

Side effects

Hot flushes (very common)
Fluid retention (very common)
Nausea (very common)
Vaginal bleeding and vaginal discharge (very common)
Skin rash (very common); thromboembolic events including DVT and pulmonary embolism (common); endometrial changes/cancer

Interactions

Anastrozole: concurrent use contraindicated
Coumarin-type anticoagulants: contraindicated in the primary prevention setting

Clinical monograph

How it works

It blocks oestrogen receptors in breast tissue while acting as a partial oestrogen agonist in other tissues such as the endometrium and bone.

Prescribing in practice

It increases the risk of venous thromboembolism and of endometrial changes/cancer — report abnormal vaginal bleeding promptly.
Hot flushes and menstrual changes are common; it is teratogenic, so non-hormonal contraception is needed.
Its activation depends on CYP2D6 — avoid strong CYP2D6 inhibitors (e.g. paroxetine, fluoxetine) where possible.

Monitoring

Review for thromboembolism and gynaecological symptoms, and check for interacting CYP2D6 drugs.

Counselling the patient

Report calf swelling or pain, or breathlessness (clot), and any unusual vaginal bleeding.
Hot flushes are common.
Use non-hormonal contraception and avoid pregnancy; tell your clinician about antidepressants.

Evidence & guidelines

Standard endocrine therapy in oestrogen-receptor-positive breast cancer and for risk reduction in higher-risk women (NICE NG101/CG164).

Reference: NICE NG101; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

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