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breast cancer and HRT

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In the Women's Health Initiative study there was a relative risk of 1.26 at 5.2 years of developing breast cancer (comparing oestrogen plus progestogen arm vs. placebo group) (1) - this meant an increased absolute risk of 8 per 10,000 women per year.

The Million Women Study (2) was set up to investigate the effects of specific types of HRT on incident and fatal breast cancer.Over one million UK women (1 084 110) aged 50-64 years were recruited into the Million Women Study between 1996 and 2001, provided information about their use of HRT and other personal details, and were followed up for cancer incidence and death. Results showed that:

  • breast cancer was more likely in current users of HRT
    • current users of HRT at recruitment were more likely than never users to develop breast cancer (adjusted relative risk 1.66 [95% CI 1.58-1.75], p<0.0001); also current users were more likely to die from breast cancer (1.22 [1.00-1.48], p=0.05). Note though that previous users of HRT were not at an increased risk of incident or fatal disease (1.01 [0.94-1.09] and 1.05 [0.82-1.34], respectively)
  • breast cancer incidence was higher in users of oestrogen-progestagen preparations than other HRT preparations
    • incidence of breast cancer was significantly higher for current users of preparations containing oestrogen only (1.30 [1.21-1.40], p<0.0001), oestrogen-progestagen (2.00 [1.88-2.12], p<0.0001), and tibolone (1.45 [1.25-1.68], p<0.0001), but the magnitude of the associated risk was much greater for oestrogen-progestagen than for other types of HRT (p<0.0001)
    • increasing total duration of use for each type of HRT resulted in an increase in risk of a current HRT user developing breast cancer:
      • 10 years' use of HRT is estimated to result in five (95% CI 3-7) additional breast cancers per 1000 users of oestrogen-only preparations
      • 10 years' of HRT in the form of oestrogen-progestagen combinations is estimated to have resulted in 19 (15-23) additional cancers per 1000 users

A study of over 100,000 women diagnosed with breast cancer in relation to type and timing of menopausal hormone therapy and breast cancer risk was published in August 2019 (3). Based on this the MRHA has stated (4) :

in the UK about 1 in 16 women who never use HRT are diagnosed with breast cancer between the ages of 50 and 69 years. This is equal to 63 cases of breast cancer per 1000 women. Over the same period (ages 50-69 years), with 5 years of HRT use, the study estimated:

  • about 5 extra cases of breast cancer per 1000 women using estrogen-only HRT
  • about 14 extra cases of breast cancer per 1000 women using estrogen combined with progestogen for part of each month (sequential HRT)
  • about 20 extra cases of breast cancer per 1000 women using estrogen combined with daily progestogen HRT (continuous HRT)

These risks are for 5 years of HRT use. The numbers of extra cases of breast cancer above would approximately double if HRT was used for 10 years instead of 5.

With respect to age of onset of use and HRT (4):

  • risk of breast cancer depends on many factors, including age at menopause
  • women who do not use HRT and who experience menopause between ages 40 and 50 years have a lower risk of breast cancer than women who experience menopause at a later age
    • however, in women who start HRT in their 40s, the number of HRT-related breast cancers diagnosed by age 69 years is similar to that in women who use HRT for a similar duration starting in their 50s
      • because women who have a menopause in their 40s have longer time as a current HRT-user plus ex-user before they are 69 years old
      • not known if the increased risk of breast cancer with HRT use is similar for women who take HRT following a premature menopause (younger than age 40 years), or how their risk may be affected by any underlying conditions

MHRA has stated (4):

  • All forms of systemic HRT are associated with a significant excess incidence of breast cancer, irrespective of the type of estrogen or progestogen or route of delivery (oral or transdermal)
  • There is little or no increase in risk with current or previous use of HRT for less than 1 year; however, there is an increased risk with HRT use for longer than 1 year
  • Risk of breast cancer increases further with longer duration of HRT use
  • Risk of breast cancer is lower after stopping HRT than it is during current use, but remains increased in ex-HRT users for more than 10 years compared with women who have never used HRT
  • Risk of breast cancer is higher for combined estrogen-progestogen HRT than estrogen-only HRT
  • For women who use HRT for similar durations, the total number of HRT-related breast cancers by age 69 years is similar whether HRT is started in her 40s or in her 50s
  • The study found no evidence of an effect on breast cancer risk with use of low doses of estrogen applied directly via the vagina to treat local symptoms

The MHRA has summarised the risks of HRT with respect to breast, endometrial and ovarian cancer (4):

Summary of HRT risks and benefits* during current use and current use plus post-treatment from age of menopause up to age 69 years, per 1000 women with 5 years or 10 years use of HRT (4)


  1. Writing Group for the Women's Health Initiative Investigators. Risks and benefits of estrogen plus progestin in healthy postmenopausal women. Principal results for the Women's Health Initiative Randomized controlled trial. JAMA 2002; 288:321-33
  2. Lancet 2003; 362: 419-27
  3. Collaborative Group on Hormonal Factors in Breast Cancer. Type and timing of menopausal hormone therapy and breast cancer risk: individual participant meta-analysis of the worldwide epidemiological evidence. The Lancet. Published August 29, 2019.
  4. MHRA (August 2019). Hormone replacement therapy and risk of breast cancer


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