Nearly all types of menopausal hormone therapy increase breast cancer risk, with some risk persisting for many years, a major new international study has found.
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. It is the most comprehensive study of its kind.
An accompanying study also finds 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 shows all types of menopause hormone therapy (MHT) - except topical oestrogens applied to the vagina - are associated with increased risks of breast cancer.
"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 increases the longer women use MHT, with a doubling in the risk of breast cancer for women using oestrogen-progestagen therapy for five to14 years.
"For women using combined oestrogen-progestogen 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 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.
"If women use MHT for less than a year, there is little increase in the risk of breast cancer."
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 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 approximately 800 breast cancers have been prevented every year in Australia due to more targeted use of MHT since 2001.
The 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.