Selective oestrogen receptor modulator (SERM)

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.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses 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.

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. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

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