A British study has found women who use the most common form of hormone replacement therapy (HRT) are about three times more likely to get breast cancer.
The study followed 39,000 women for six years and found that up until now, the risk has been "understated".
Interestingly the cancer was only an issue for women using the combined oestrogen and progestogen HRT- not those who used oestrogen-only HRT.
British woman Pat Sutherland is battling terminal breast cancer and for nine years in the 1980s she took the combined HRT pill.
"I was assured at the time that it was a kind of wonder drug, that it could do everything from curing menopausal symptoms to keeping our hearts and skin healthy," she says.
HRT is used to treat the symptoms of menopause, which include hot flushes and night sweats, by increasing a woman's oestrogen levels.
Scientists at the Institute of Cancer Research say women using combined HRT for five years are nearly three times more likely to develop breast cancer than women not using it - or those taking oestrogen only.
The New Zealand Breast Cancer Foundation says the risks have always been known, but not to what extent.
"This latest study has shown the risk to be much much higher than studies before," says CEO Evangelia Henderson.
"So we all need to be mindful of that and just take heed of what the numbers are showing us."
The study found women who spent more than 15 years taking combined HRT were 3.3 times more likely to develop breast cancer.
Ms Henderson says the issue for women is to reduce the length of treatment as short as possible.
"It's really important for menopausal woman to be aware of the risks as well as course the benefits of HRT, and to have a really good conversation with their doctors and make an informed choice."
The study found the risk of breast cancer declined when women stopped taking HRT.