# IBIS-I (International Breast Intervention Study I)

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

## TL;DR

The UK-led prevention trial that showed five years of tamoxifen keeps preventing breast cancer for at least twenty years, and the reason NICE tells the NHS to offer it to premenopausal women at high familial risk.

## Summary

IBIS-I randomised 7,154 women at increased risk of breast cancer to tamoxifen or placebo for five years. At a median 16 years of follow-up, breast cancer (invasive plus ductal carcinoma in situ) occurred in 251 women in the tamoxifen arm and 350 in the placebo arm, a hazard ratio of 0.71, with the reduction as large after ten years as during the first ten; the effect was confined to oestrogen-receptor-positive disease and there was no difference in breast cancer mortality. Endometrial cancer and thromboembolism were increased mainly while the drug was being taken. With the American NSABP P-1 trial it is the evidence behind NICE CG164 recommendation 1.7.21, which tells the NHS to offer tamoxifen for five years to premenopausal women at high risk.

## Fields

- Kind: Trial
- Status: positive
- Last checked: 2026-09-25
- Also known as: IBIS-1; ISRCTN91879928
- Tags: uk; breast; prevention
- Registry id: ISRCTN91879928
- Phase: 3
- Setting: Prevention: tamoxifen 20 mg daily for five years against placebo in women aged 35 to 70 at increased risk of breast cancer
- Sponsor: Queen Mary University of London (Cancer Research UK funded)
- Enrolled: 7154
- Result: Breast cancer in 251 of the tamoxifen arm against 350 of the placebo arm at a median 16 years, hazard ratio 0.71, with no difference in breast cancer mortality (Lancet Oncology 2015).

## Sources

- Lancet Oncology 2015 (DOI): https://doi.org/10.1016/S1470-2045(14)71171-4
- ISRCTN91879928: https://www.isrctn.com/ISRCTN91879928
- 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: [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-i.json