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 progesterone .
Discovery made by scientists from one of largest ever studies into HRT .
But women taking oestrogen-only HRT may actually have a reduced risk .