Wed. Jul 24th, 2024
Note: Hormone therapy (HT) may also be referred to as hormone replacement therapy (HRT).

Background information

The Million Women Study (MWS) is a UK national observational study of women’s health, involving around one million women aged 50 and over presenting for breast screening. It aims to answer many outstanding questions about the factors affecting women’s health in this age group. The main focus of the study relates to the effects of hormone replacement therapy use.


In the Lancet, 9 August 2003 results were reported for a study of over one million UK women aged 50-64 years who had provided information about their use of hormone therapy (HT) prior to a screening mammogram and were followed up for cancer incidence and death.


The Million Women Study published in the Lancet
Breast Cancer and hormone-replacement therapy in the Million Women Study
Lancet 2003; 362419-427



The study reports findings in relation to risk of breast cancer for women on combined oral therapy very similar to those of the WHI study in 2002 and 2003.

These findings provide further information about the risk of breast cancer with hormone therapies.

The authors of the Lancet study presented the following calculations:

  • For 1,000 women who go through menopause aged 50 and who do not take HT, 27 breast cancers would be expected to be diagnosed by the age of 55 years.
  • If the 1000 women had five years of combined oral HT, a total of 34 breast cancers (an extra seven cancers) would be expected.

This is the first report of an increased risk of breast cancer for women taking oestrogen-alone (oral, skin patch or implant), progesterone or tibolone. It should be noted that there is evidence that tibolone was being prescribed preferentially to women in the UK considered at increased breast cancer risk.

The authors estimated that, if 1000 women took oestrogen alone for five years, there would be a total of 28.5 cases of breast cancer (an extra 1.5 cases per 1,000). It should be noted that in the WHI randomised controlled trial of oestrogen-alone, with its much stronger trail design, there was evidence for a decreased risk of breast cancer in the first five years of therapy.

An increase in risk, similar to that of oestrogen only was also observed for progesterone alone and tibolone.

In a previous publication, the authors themselves have urged people to be cautious in interpreting the findings from this study. This is because the design of the study is weaker than the WHI randomised trial so the conclusions are not as reliable.

The commentary accompanying the Lancet publication describes HT as preventive therapy. This is not the current recommended use of HT. Hormone therapy in Australia is prescribed for the management of symptoms that significantly impair a woman’s quality of life. Based on the findings of this study continued use for this purpose remains appropriate where the woman is fully informed about the associated risks.

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