While the link between breast cancer and menopausal hormone therapy has been well-documented it was not clear until now that the elevated risk of the disease can last years after women stop undergoing the treatment. Without hormone therapy, women in western countries have a 6.3 per cent chance of developing the disease between the ages of 50 and 69. The study found, however, the risk increased by a further two per cent for those using oestrogen and daily progestogen therapy for five years starting at age 50 and 1.4 per cent for those using oestrogen and intermittent progestogen. One of the study's researchers, Australian National University Professor Emily Banks, said the study showed all types of menopause hormone therapy – except topical oestrogen applied to the vagina – are linked with an increased risk of breast cancer.

“Unfortunately, the news isn’t good," Professor Banks said. “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." Professor Banks said the risk increased the longer women use MHT, with a doubling in the risk of breast cancer for women using oestrogen-progestogen therapy for five to 14 years. The study also shows the risks of breast cancer can last for years after MHT use. Professor Emily Banks “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," Professor Banks said. "These findings mean that the overall risks are greater than previously thought."

Gynecologist Raewyn Teirney, 56, underwent MHT for five years before being diagnosed with breast cancer last year. Hormone therapy is currently the most effective treatment available for menopause symptoms. Gynaecologist Raewyn Teirney, 56, underwent MHT for five years before being diagnosed with breast cancer last year. She was aware of the risks but opted to go ahead with MHT to alleviate debilitating menopausal symptoms including sleeplessness and hot flushes which had failed to respond to other treatments. “As a busy doctor it relieved the symptoms immediately and helped me continue working and being able to care for my patients," she said.

Dr Teirney remained extra vigilant. When she found a lump in her breast she got it checked immediately and finishes chemotherapy next week. "Do I regret being on MHT? Possibly, I regret being on it for as long as I was," she said. "This will really scare women and they'll probably want to stop treatment but I think they shouldn't stop until they speak to their doctor about their options." More than 300,000 Australian women are using hormone replacement therapy. It is estimated there are about 12 million women using it in Western countries. “The study confirms the information that we’ve had in previous studies, but in the most scientific, robust and rigorous way we’ve had to date,” oncologist Professor Kelly-Anne Phillips said.

When asked if women should avoid MHT due to the increased risk of cancer, Professor Phillips who is the founder of the Peter Mac Breast and Ovarian Cancer Risk Management Clinic, said the decision should be made on a case-by-case basis. Loading “It’s very hard to make a blanket suggestion for all women across the population because as women and as clinicians we need to take into account individual risk factors [including a family history of the disease] and a women's preference for treatment to make those decisions in a collaborative fashion," she said. Professor Phillips said the study suggested that using MHT for a short duration (less than a year) was relatively safe. "Some women will find short term it can help relieve their symptoms," she said.

Professor Phillips, warned however, that women who have been on MHT for a year should have their treatment reviewed, adding there were a number of alternatives including weight loss, moisturisers for vaginal dryness and avoiding caffeine or alcohol. Professor Banks said the study’s findings were an important public health warning. “Women should only use MHT to manage moderate to severe menopausal symptoms and not for the prevention of disease,” Professor Banks said. "Therapy should be used for as short a time as possible.” The study was conducted by researchers from the Collaborative Group on Hormonal Factors in Breast Cancer, which spans more than 100 institutions worldwide, including the Australian National University and the Cancer Council. Pink Hope founder Krystal Barter said the findings allowed women to have an "honest dialogue" with their doctors about the risks involved with the therapy and weigh up their options.