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Wolfson Institute of Preventive Medicine - Barts and The London

Anastrozole protects against breast cancer for at least 12 years

A study led by Wolfson Institute Director Professor Jack Cuzick has confirmed that Anastrozole offers significant long-term protection for postmenopausal women who take it for five years. Findings from the IBIS-II trial published in the Lancet, and simultaneously presented at the San Antonio Breast Cancer Symposium in Texas, show that in trial participants 12 years after starting Anastrozole (up to seven years after they last took the drug), breast cancer incidence was 49% lower than in women given a placebo.

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Professor Cuzick said: “This exciting finding makes a strong case for anastrozole being the drug of choice for post-menopausal women at high risk of developing breast cancer. Previous research confirmed that anastrozole is very effective while women are still taking the drug, but this is the only trial looking at whether it offers long-term protection for women at high risk of developing breast cancer.”

The 49% reduction achieved 12 years after women started taking anastrozole (seven years after stopping) is significantly higher than the 28% decrease in breast cancer recorded for women who take tamoxifen. The latest report also confirms that anastrozole does not have the same type of long term side effects associated with tamoxifen, including endometrial cancer.

Anastrozole inhibits the production of oestrogen in postmenopausal women. The first phase of this study, published in 2013, established its value as a prophylactic therapy. It reported that breast cancer occurrence amongst women taking the drug fell by 53%. Anastrozole was subsequently recommended for preventive use by NICE in the UK in 2017 and by the US Preventive Services Task Force (USPSTF) in 2019.

Professor Cuzick said that with few deaths from breast cancer among trial participants, longer follow-up will be needed to evaluate whether anastrozole can reduce deaths from the disease.

Astra Zeneca, Cancer Research UK and the National Health and Medical Research Council Australia funded the research.


Jack Cuzick, Ivana Sestak, John F Forbes, Mitch Dowsett, Simon Cawthorn, Robert E Mansel, Sibylle Loibl, Bernardo Bonanni, Gareth Evans, Anthony Howell, on behalf of the IBIS-II investigators. Use of anastrozole for breast cancer prevention (IBIS-II): long-term results of a randomised controlled trial. Lancet 2019.