The results of the UK Million Women Study have recently been published in The Lancet in August 2003. They confirm and expand on the results of the previous Women's Health Initiative (WHI) study which investigated only combination hormone replacement therapy (HRT).

The Million Women Study is a cohort study investigating the effects of specific types of HRT on breast cancer incidence and mortality. More than a million women aged 50 – 64 years were recruited between 1996 and 2001 in UK and half the women had used HRT. A summary of the key findings is included below:

• Among the postmenopausal women (n = 828,923) included in the main analysis, the risk of breast cancer was significantly higher among ever users than never users of HRT at baseline [relative risk of 1.43; 95% CI (1.36 – 1.5)]. However, among the ever users of HRT, the relative risk was increased in current users [1.66 (1.58 – 1.75)] but not in past users [1.01 (0.94 –1.09)]. The study showed that the risk of breast cancer begins to decline when HRT is stopped and by 5 years reaches the same level as women who have never taken HRT.
  
  
• In current users, the risk of breast cancer increased with increasing total duration of use, regardless of HRT type. The relative risk of breast cancer was significantly raised for current users of oestrogen only preparations [relative risk of 1.30 (1.21 – 1.40)], tibolone [1.45 (1.25 – 1.68)], and oestrogen-progestagen combinations [2.00 (1.88 – 2.12)]. This study demonstrated a higher risk of breast cancer associated with the use of combined HRT than oestrogen only therapy.
  
  
• For oestrogen only preparations, the relative risk of breast cancer was significantly raised separately for users of oral, transdermal and implanted formulations compared to never users of HRT [1.32 (1.21 – 1.45), 1.24 (1.11 – 1.39) and 1.65 (1.26 – 2.16), respectively].
  
  
• With 4.1 years of follow-up for mortality, the relative risk of death from breast cancer was increased in current users of HRT at recruitment [1.22 (1.00-1.48)] compared with never users.
  
  
•  The estimated number of extra cases of breast cancer occurring after 5 and 10 years using oestrogen only and combined HRT are shown in Table 1.

Table 1: Estimated excess incidence of invasive breast cancer in 1,000 women who had ever used HRT relative to never users. Baseline incidence of invasive breast cancer in never users is ~ 32 per 1,000 between the ages of 50 and 65 years.