A new study has shown women still have an increased risk of breast cancer years after stopping taking hormone replacement therapy . Hormone replacement therapy increases a woman 's risk of breast cancer up to eight years after she has stopped treatment , scientists have warned . The new findings come from experts who conducted one of the world 's largest studies into the effects of HRT - used to ease the effects of menopause . The Women 's Health Initiative in the US and the Million Women Study in the UK have both revealed strong evidence that HRT increases a woman 's risk of breast , womb and ovarian cancers . The Million Women Study , a collaboration between Cancer Research UK and the NHS , results suggested that women who are using combined HRT have double the risk of breast cancer compared to non-users . And researchers found if women use HRT for more than 10 years , their risks are even higher . Now scientists involved in the Women 's Health Initiative study have warned , after further follow-ups , that women who take the combined oestrogen and progesterone HRT therapy continue to have a high risk of breast cancer , up to eight years after they stop treatment . After going through the menopause a woman 's ovaries stop producing the hormone oestrogen . It triggers menopausal symptoms including hot flashes and mood swings . HRT treats the body by introducing female sex hormones . There are three types , oestrogen-only HRT , cyclical HRT - where oestrogen is taken continuously but progesterone is given either monthly or every three months - and continuous combined HRT , where both female sex hormones are taken at the same time . In the early 1990s , some smaller studies suggested the therapy would reduce a woman 's risk of coronary heart disease and osteoporosis . To examine whether that was the case , the Women 's Health Initiative in the US was tasked with investigating . They enrolled more than 26,000 women aged 50 to 79 in the study . Meanwhile , in the UK another key study , the Million Women Study , explored the effects of HRT on women aged 50 or over . Both concluded that women prescribed HRT were at a higher risk of breast cancer , than those not receiving the treatment . WHI researchers had intended to follow the women up until 2005 , but in 2002 it became clear women taking HRT were suffering higher rates of heart disease . Those women who took the combined HRT , which boosts the body 's levels of both female sex hormones oestrogen and progesterone , have an increased risk of breast cancer up to eight years after treatment . They discovered it was the women taking either the combination HRT or those taking oestrogen-only treatment that were going on to suffer heart problems . As a result , all women were asked to stop taking their treatment - HRT or placebo - immediately . After the reported increased risk in breast cancer for women taking combined HRT , use of the therapy dropped dramatically . The new study , led by Dr Rowan Chlebowski , an oncologist at Los Angeles Biomedical Research Institute , and one of the initial investigators on WHI , focused on a woman 's risk of breast cancer after stopping HRT treatment . He told Time : 'It looks like hormones have longer term lingering effects . 'For oestrogen and progesterone together , we see an increase in risk - even years after you stop . 'But for oestrogen alone , it looks like the hormone may be more favourable in reducing breast cancer risk than we thought before . 'The oestrogen alone findings are now quite compelling that we may have to call it risk reduction . ' In total , researchers examined 27,347 women aged 50 to 79 years old , who had been through the menopause . They had all taken part in two previous WHI studies , having enrolled at 40 centres across the US from 1993 to 1998 . The Women 's Health Initiative in the US and the Million Women Study in the UK are the two largest studies into the effects of HRT . Both found strong evidence the treatment increases a woman 's risk of breast cancer , pictured , with the latter finding it doubled a woman 's risk . In the first two years , after stopping HRT treatment , researchers noted a sharp decrease in breast cancer risk in those women who had taken both oestrogen and progesterone . But the study authors noted : 'Subsequently , breast cancer risk increased throughout the intervention phase and became statistically significant . ' Even up to five-and-a-half years later , breast cancer risk remained high . In addition , the researchers note that at an eight-year follow-up a 'risk reduction was not observed ' . But for those women taking oestrogen only treatment , the risk for invasive breast cancer fell . However , despite the findings , Dr Chlebowski urged caution . He said while the results do show a higher than normal risk of breast cancer years after stopping HRT , it is only ' a little more risk than we thought with oestrogen and progesterone ' . 'But it 's always difficult to figure out how to categorise that risk , ' he added . 'It 's different for each woman . '
HRT increase risk of disease up to eight years after stopping treatment . Applies to those taking combined HRT - oestrogen and progester27,347 . Discovery made by scientists from 27,347 of largest ever studies into HRT . But women taking oestrogen-only HRT may actually have a reduced risk .