# Chemoprevention (tamoxifen, anastrozole, raloxifene) and ER-negative breast cancer

Source: https://onco.cc/terms/chemoprevention-and-er-negative-breast-cancer/  
OnCo record `chemoprevention-and-er-negative-breast-cancer` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Tamoxifen and anastrozole taken for five years cut the number of new breast cancers in women at raised risk by a third to a half, and NICE offers them to women at high or moderate risk. But the cancers they prevent are oestrogen-driven ones; in the trials they made no difference to oestrogen receptor-negative cancers, the kind BRCA1 carriers mostly get.

## Summary

NICE CG164 offers tamoxifen for five years to premenopausal women at high risk of breast cancer (unless they have a history or increased risk of thromboembolic disease or endometrial cancer), anastrozole for five years to postmenopausal women at high risk (unless they have severe osteoporosis), tamoxifen or off-label raloxifene as alternatives, considers the same drugs at moderate risk, and does not offer chemoprevention after bilateral risk-reducing mastectomy. The trials behind those offers were run in women defined by family history or age-related risk, and their effect was confined to ER-positive disease. IBIS-I randomised 7,154 women aged 35 to 70 at increased risk to tamoxifen 20 mg or placebo for five years; after a median 16 years, breast cancer occurred in 7.0 against 9.8 percent (hazard ratio 0.71), with the reduction similar in years 0 to 10 and after 10 years, the greatest effect on invasive ER-positive cancer (0.66) and ductal carcinoma in situ (0.65), and no effect on invasive ER-negative cancer (1.05) (Cuzick 2015). NSABP P-1 randomised 13,388 women to tamoxifen or placebo; tamoxifen reduced invasive breast cancer by 49 percent and ER-positive tumours by 69 percent, with no difference in ER-negative tumours, at the cost of more endometrial cancer (risk ratio 2.53), stroke, pulmonary embolism and deep-vein thrombosis, mainly in women 50 or older (Fisher 1998). IBIS-II randomised 3,864 postmenopausal women at increased risk to anastrozole 1 mg or placebo for five years; after a median 131 months breast cancer fell by 49 percent (85 against 165 cases), invasive ER-positive cancer by 54 percent and ductal carcinoma in situ by 59 percent, with no excess of fractures or cardiovascular disease and no difference in deaths (Cuzick 2020). Cancer Research UK notes that the evidence is unclear for women with gene changes. For carriers who have already had a breast cancer, pooled observational cohorts found tamoxifen associated with a lower risk of contralateral breast cancer in BRCA1 (hazard ratio 0.38) and BRCA2 carriers (0.33), not differing by the ER status of the first cancer, though the prospective-only estimates were weaker (0.58 and 0.48) (Phillips 2013). No drug is proven to prevent ER-negative or triple-negative breast cancer; surveillance, risk-reducing surgery and breastfeeding (Palmer 2014) are what the evidence supports for BRCA1 carriers.

## Fields

- Kind: Term
- Last checked: 2026-09-24
- Also known as: Breast cancer chemoprevention; Tamoxifen for prevention; Anastrozole for prevention; IBIS-I; IBIS-II; NSABP P-1; Preventive tamoxifen and BRCA1
- Tags: breast; tnbc

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Tamoxifen
- NICE CG164: familial breast cancer (referral, genetic testing, surveillance, risk-reducing surgery, chemoprevention): https://www.nice.org.uk/guidance/cg164/chapter/Recommendations
- Cuzick, Lancet Oncol 2015: IBIS-I tamoxifen prevention trial, extended long-term follow-up: https://doi.org/10.1016/s1470-2045(14)71171-4
- Fisher, JNCI 1998: tamoxifen for prevention of breast cancer, NSABP P-1: https://doi.org/10.1093/jnci/90.18.1371
- Cuzick, Lancet 2020: anastrozole for breast cancer prevention (IBIS-II), long-term results: https://doi.org/10.1016/s0140-6736(19)32955-1
- Phillips, J Clin Oncol 2013: tamoxifen and contralateral breast cancer risk in BRCA1 and BRCA2 carriers: https://doi.org/10.1200/jco.2012.47.8313
- CRUK: family history and inherited genes in breast cancer: https://www.cancerresearchuk.org/about-cancer/breast-cancer/risks-causes/family-history-and-inherited-genes
- Palmer, JNCI 2014: parity, lactation and breast cancer subtypes in African American women (AMBER): https://doi.org/10.1093/jnci/dju237

## Connected records

- cancers: [BRCA-associated triple-negative breast cancer](https://onco.cc/cancers/brca-associated-tnbc/), [Breast cancer (all types)](https://onco.cc/cancers/breast-cancer/), [HR-positive / HER2-negative breast cancer](https://onco.cc/cancers/breast-hr-positive/), [Triple-negative breast cancer (TNBC)](https://onco.cc/cancers/tnbc/)
- technologies: [Chemoprevention & risk-reducing surgery](https://onco.cc/technologies/chemoprevention/), [Endocrine therapy (SERMs, AIs, SERDs)](https://onco.cc/technologies/endocrine-therapy/)
- targets: [Estrogen receptor (ERα)](https://onco.cc/targets/estrogen-receptor/)
- drugs: [Anastrozole](https://onco.cc/drugs/anastrozole/), [Raloxifene](https://onco.cc/drugs/raloxifene/), [Tamoxifen](https://onco.cc/drugs/tamoxifen/)
- terms: [Risk-reducing surgery for BRCA carriers (bilateral and contralateral mastectomy, salpingo-oophorectomy)](https://onco.cc/terms/risk-reducing-surgery-brca-carriers/)

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