Hormone Replacement Therapy Risk of Breast Cancer - more confirmation

Menopause News March 13th 2011 

It's already well known that the hormone replacement therapy risk of breast cancer increases with the length of time that you use hormones(1). But does it make a difference at what stage of your life, relative to your menopause, that you start taking hormones?

A study published recently in the Journal of the National Cancer Institute (JNCI), has found that timing is all important(2).

The main new finding from this study was that breast cancer risk is increased if you start hormone replacement therapy before the menopause or within 5 years of menopause.

The researchers, from the UK's Million Women Study, found that breast cancer risk was increased for all hormone preparations: estrogen-progestin in combination, estrogen only and tibolone - a synthetic steroid licensed in Europe but not the USA.

A woman who starts hormones before or within 5 years of her menopause has roughly double the chances of getting breast cancer compared with a woman who has never used HRT.

In this study, just over half (55%) of the over 1 million women studied, had ever used hormones and about a third were current users. Other big studies, such as the Women's Health Initiative in the US(3), have also found an increase in breast cancer in women taking HRT, but the importance of timing has been less clear until now.

Both the US trial and this UK study show that women who take the combined hormone preparation of estrogen-progestin are at greater risk than women taking estrogen only. But remember, only women who have had a hysterectomy can safely take estrogen without progestin - otherwise you increase your risk of getting cancer of the uterus.

But before you panic, there is some consoling news: about 2 years after stopping HRT, the risk of breast cancer returns to the same level as that of women who have never taken hormones. On the other hand, women who take hormones for more than 5 years stand to increase their risk, the longer they use them.

There are differences in the hormone replacement therapy risk, depending on which preparation you use. For instance women who began therapy late - at least 5 years after menopause - and took the estrogen-only preparation, had no increase in risk.

Another factor to be aware of is obesity: your risk of breast cancer increases if you are overweight or obese, regardless of whether or not you take hormones.

In the Million Women study, women whose body mass index (BMI) was over 35kg/m2 and who had never used hormones, had nearly the same increase in risk as women on estrogen-only hormone therapy. But women taking the combined hormone preparation had the greatest risk of breast cancer and if they are normal weight (BMI less than 25), then the proportionate increase in hormone replacement therapy risk, is greater.

The Million Women Study holds observational data on an incredible, one in four of all UK women aged 50 - 64 years old when they were recruited, between 1996 and 2001. Women attending the UK National Health Service breast cancer screening service were asked to fill in a questionnaire about their hormone use - amongst other things.

You can find out more about the Million Women Study by clicking the link below.


  1. Chlebowski RT, Anderson GL. The Influence of Time from Menopause and Mammography on Hormone Therapy-Related Breast Cancer Risk Assessment.J Natl Cancer Inst. 2011 Feb 16;103(4):284-5. Epub 2011 Jan 28.
  2. Beral V, Reeves G, Bull D, Green J for the Million Women Study Collaborators. Breast Cancer Risk in Relation to the Interval Between Menopause and Starting Hormone Therapy. J Natl Cancer Inst. 2011 Feb 16;103(4):296-305. Epub 2011 Jan 28.
  3. The Million Women Study
  4. Chlewbowski et al. Breast cancer in postmenopausal women after hormone therapy. JAMA 2011; 305:466-7.

Published March 13th 2011.

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