# IBIS-II (International Breast Intervention Study II)

Source: https://onco.cc/trials/ibis-ii/  
OnCo record `ibis-ii` (Trial). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

The trial that made anastrozole a prevention drug, halving breast cancer in postmenopausal women at high risk and, with a licence granted in 2023, putting a generic tablet on the NHS prevention pathway.

## Summary

IBIS-II randomised 3,864 postmenopausal women at increased risk of breast cancer to anastrozole or placebo for five years. After a median 131 months, 85 women in the anastrozole arm and 165 in the placebo arm had developed breast cancer, a hazard ratio of 0.51, with the benefit continuing after treatment stopped and no excess of fractures or cardiovascular disease in the post-treatment period. NICE CG164 recommendation 1.7.22, amended in 2023, tells the NHS to offer anastrozole for five years to postmenopausal women at high risk unless they have severe osteoporosis; the MHRA licensed anastrozole for prevention in 2023 through its repurposing programme.

## Fields

- Kind: Trial
- Status: positive
- Last checked: 2026-09-25
- Also known as: IBIS-2; IBIS-II Prevention; ISRCTN31488319
- Tags: uk; breast; prevention
- Registry id: ISRCTN31488319
- Phase: 3
- Setting: Prevention: anastrozole 1 mg daily for five years against placebo in postmenopausal women at increased risk of breast cancer
- Sponsor: Queen Mary University of London (Cancer Research UK funded)
- Enrolled: 3864
- Result: Breast cancer in 85 of 1,920 on anastrozole against 165 of 1,944 on placebo after a median 131 months, hazard ratio 0.51 (Lancet 2020).

## Sources

- Lancet 2020 (DOI): https://doi.org/10.1016/S0140-6736(19)32955-1
- ISRCTN31488319: https://www.isrctn.com/ISRCTN31488319
- NICE CG164 recommendations: https://www.nice.org.uk/guidance/cg164/chapter/Recommendations

## Connected records

- cancers: [Breast cancer (all types)](https://onco.cc/cancers/breast-cancer/)
- drugs: [Anastrozole](https://onco.cc/drugs/anastrozole/), [Tamoxifen](https://onco.cc/drugs/tamoxifen/)
- people: [Jack Cuzick](https://onco.cc/people/jack-cuzick/)

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JSON: https://onco.cc/api/v1/entities/ibis-ii.json