Hormone replacement therapy containing only oestrogen may slash the risk of breast cancer by 23 per cent, scientists have said.
However, those taking combined HRT with progesterone have a 29 per cent higher risk of the disease, with the effects persisting decades after use.
The major study involved more than 27,000 women in the US who were randomly assigned to a treatment or placebo.
Their health was tracked for at least 16 years after they used HRT – which is given to reduce women’s menopause symptoms.
Until now, health officials have warned that both types of HRT can cause breast cancer but that the risks are thought to be bigger with combined HRT.
This was backed by a bombshell Oxford University study in August, which said any type of HRT could raise the risk of breast cancer by up to a third.
Experts said the latest findings – which go against previously held beliefs – are ‘surprising’ but have reassured women they don’t need to stop taking HRT.
HRT containing only oestrogen may slash the risk of breast cancer by 23 per cent, scientists have said, while combined HRT increases the risk
Up to one million women in Britain take HRT medication every year, with around 360,000 prescriptions dispensed every month.
Oestrogen-only HRT can increase the risk of womb cancer, and therefore is only recommended for women who have previously had a hysterectomy.
But this research suggests it may lower the risk of breast cancer at the same time.
This study compared an oestrogen-only pill, available as a product called Premarin, and a progesterone one, given under brand names Provera or Depo-Provera.
Women in two studies were all between 50 and 79 years old, never had breast cancer, and randomly took a type of HRT or a placebo.
WHAT ARE THE DIFFERENT TYPES OF HRT?
HRT replaces the hormones that a woman’s body no longer produces because of the menopause.
The two main hormones used in HRT are:
- Oestrogen – types used include estradiol, estrone and estriol
- Progestogen – a synthetic version of the hormone progesterone, such as dydrogesterone, medroxyprogesterone, norethisterone and levonorgestrel
HRT involves either taking both of these hormones (combined HRT) or just taking oestrogen (oestrogen-only HRT).
Most women take combined HRT because taking oestrogen on its own can increase your risk of developing womb cancer. Taking progestogen alongside oestrogen minimises this risk.
Oestrogen-only HRT is usually only recommended for women who have had their womb removed during a hysterectomy.
Among women who took the combined HRT pill, the risk of breast cancer was 29 per cent higher than taking no pill (there were 1,003 cases in a group of 8,506 after 18 years).
But for women who took the oestrogen-only therapy, the risk actually dropped by 23 per cent compared to no pill (520 cases in a group of 5,310 after 16 years), the study found.
One in eight women taking both oestrogen and progesterone got cancer, therefore, compared to one in 10 women taking oestrogen only.
Women taking oestrogen were also less likely to die from breast cancer than those taking the placebo, the study found.
The research has not been published but is being presented at the San Antonio Breast Cancer Symposium in Texas today.
The team did not offer an immediate explanation for why this study found a surprisingly lower risk of breast cancer with oestrogen.
Exposure to oestrogen for a long time or in high levels, due to starting puberty early or the menopause late, for example, is known to contribute to and fuel cancer.
In the past scientists have said progesterone increases the number of potentially-cancerous cells which are produced in the breast, raising the risk of tumours.
But the effects of oestrogen-only therapy were found to be less dramatic.
Dr Rowan Chlebowski, chief of the Division of Medical Oncology and Hematology at Harbor-UCLA Medical Center, led the new study.
He said: ‘Menopausal hormone therapy with oestrogen plus progestin [progesterone] and oestrogen alone continues to be used by millions of women worldwide.
‘Nonetheless, after nearly half a century, menopausal hormone therapy influence on breast cancer incidence and mortality remains unsettled, with discordant findings from prospective observational studies compared to findings from randomized clinical trials.’
Early results did not appear to clarify cancer rates among the women who took placebo pills.
Dr Chlebowski said: ‘In contrast to decades of observational study findings… [oestrogen] alone significantly reduced breast cancer incidence and significantly reduced deaths from breast cancer, with these favorable effects persisting over a decade after discontinuing use.’
Dr Melanie Davies, consultant gynaecologist at University College London Hospitals (UCLH), who was not involved in the study, said: ‘The finding of benefit from oestrogen-only HRT is surprising.’
Despite the findings, Dr Davies said women should not change or stop taking their HRT treatment.
She added: ‘The type of HRT used in the study, which started in 1993, is no longer available in UK.
‘Modern forms of HRT may have different safety profiles – hopefully better, as modern HRT is more like the body’s own hormones.’
It’s not first time found oestrogen-only HRT has been linked to lower odds of breast cancer – another study found the same 23 per cent lower risk in a study of 7,500 American women.
The NHS is clear that women who take HRT for more than one year have a higher risk of breast cancer than women who never use HRT.
It says: ‘The risk is linked to all types of HRT except vaginal oestrogen.’
Cancer Research UK also warns all types of HRT increase risk of breast cancer, but more so for combined HRT.
Earlier this year, research by Oxford showed both types of HRT were linked with significantly increased risk of breast cancer.
Based on 58 global studies, the research suggested that HRT causes around one in 20 cases of breast cancer – nearly 3,000 a year in Britain.
UCL’s Dr Davies suggested this study, finding breast cancer risk differs between HRT types, was more robust.
The UK is still in the grip of a nationwide HRT shortage which has affected tens of thousands in recent months.
The British Menopause Society said in November there ‘continues to be availability issues with many HRT products related to manufacturing shortages or supply issues’.
WHAT IS HORMONE REPLACEMENT THERAPY?
WHAT IS HRT AND WHAT DOES IT DO?
HRT does the work of oestrogen, levels of which plummet after the menopause. Women usually take a combination of synthetic oestrogen and a second hormone, progesterone.
‘Most women in the UK take combined HRT because taking oestrogen on its own can increase the risk of developing cancer of the womb,’ says Kathy Abernethy, chair of the British Menopause Society. ‘Oestrogen-only HRT is usually only given to women who have had their wombs removed.’
ARE THERE ANY RISKS TO CONSIDER?
A major U.S. study in 2002, from the Women’s Health Initiative USA, was the first to ring alarm bells that HRT may lead to an raised risk of heart disease and breast cancer. As a result, many doctors stopped prescribing it overnight.
But the study was found to be flawed — the average age of the women in the study was 63, when the risk of breast cancer naturally rises anyway, and half were smokers.
‘The risks were overestimated for women of normal menopausal age between 50 and 60,’ says Kathy. ‘For most women under the age of 60, and for many over age 60, the benefits of HRT are clear.’
SO DOES HRT REALLY CAUSE CANCER?
Any risk comes with longer use, says Kathy.
Cancer Research UK says there is strong evidence HRT can cause breast, womb and ovarian cancer, but the chance is low compared to other risk factors. To put it in perspective, while minimising HRT could prevent 1,400 cancer deaths per year, keeping to a healthy weight could prevent 13,200 and stopping smoking could prevent 22,000.
ARE THERE OTHER SIDE-EFFECTS?
‘Women who take HRT may have side-effects including breast tenderness, headaches, nausea, indigestion, tummy pain and vaginal bleeding,’ says Professor Kamila Hawthorne, Royal College of GPs’ professional development vice-chair. Taking HRT as tablets (not patches or gels) may slightly raise the risk of blood clots.
WHO SHOULD NOT BE GIVEN HRT?
Those who have a personal or family history of hormone-sensitive cancers, such as ovarian and breast, and women who have had deep vein thrombosis. High blood pressure should be controlled before starting HRT.
WHAT ARE THE ALTERNATIVES?
Non-hormonal options include Tibolone (Livial), derived from the Mexican yam, which mimics oestrogen. Blood pressure medication Clonidine, which affects the dilation of blood vessels, can alleviate hot flushes and night sweats.
Bio-identical hormones, derived from plant oestrogens and prescribed by private clinics, are said to be similar to human sex hormones. But he NHS does not recommend these as they are not regulated.