Another red flag has been raised over menopausal hormone therapy with the most comprehensive study of its kind showing that it increases the risk of breast cancer, even years after treatment stops.
The study, involving researchers from the Australian National University (ANU) and Cancer Council NSW, examined data from more than 100,000 women with breast cancer from 58 different studies worldwide.
An accompanying study has also found that users of menopausal hormone therapy, also known as HRT, face an increased risk of dying from breast cancer.
ANU Professor Emily Banks said the new study showed all types of menopause hormone therapy (MHT) – except topical oestrogens applied to the vagina – are associated with increased risks of breast cancer.
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“As well as providing more accurate evidence on the relationship of menopausal hormone therapy and breast cancer, these new findings address three key unanswered questions: breast cancer risks related to less than five years of use, how long risks last for after use stops and the risks of dying from breast cancer,” Professor Banks said.
“Unfortunately, the news isn’t good. Our findings show women using the most common kind of menopausal hormone therapy (MHT) for one to four years have a 60 per cent increase in the risk of breast cancer, compared to women who have never used it.”
Risk increased the longer women use MHT, with a doubling in the risk of breast cancer for women using oestrogen-progestagen for five to 14 years.
For women using combined oestrogen-progestagen MHT for five years starting at age 50, around one in 60 will develop breast cancer because of the therapy. The figure is about one in 200 for women using oestrogen-only menopausal hormone therapy.
The study also shows risks of breast cancer last for years after MHT use.
“Our study also shows the risks of MHT-caused breast cancer gradually decrease after women stop using these therapies, but some increased risk persists for more than a decade after use stops. These findings mean that the overall risks are greater than previously thought,” Professor Banks.
But if women used MHT for less than a year, there was little increase in the risk of breast cancer, she said.
According to the study, increases in breast cancer risk are about twice as great for women who use MHT for 10 years rather than five years. MHT also increases the risk of stroke, venous thrombosis and ovarian cancer but reduces the risk of hip fractures, which occur later in life.
There are about 12 million women using MHT in Western countries, and over 300,000 current users in Australia.
Professor Banks said the new study’s findings provided an important public health warning.
“A key take-home message from this study is that women should only use MHT to manage moderate to severe menopausal symptoms and not for the prevention of disease,” Professor Banks said. “And therapy should be used for as short a time as possible.”
“Check in with your doctor about whether you need to continue using MHT, especially if you have been on it for over a year.”
Past Cancer Council/ANU research showed that about 800 breast cancers have been prevented every year in Australia due to more targeted use of MHT since 2001.
Nearly 20,000 Australian women will be diagnosed with breast cancer this year, and more than 3000 will die from the disease, according to Australian Government estimates.
The ACT has the highest rate of breast cancer in the country, according to Australian Institute of Health and Welfare.
The new study was conducted by researchers from the Collaborative Group on Hormonal Factors in Breast Cancer, from over 100 institutions worldwide, including ANU and Cancer Council NSW.
The findings are published in Lancet.