The risks of hormone replacement therapy (HRT) may have been underestimated, UK experts say.
The new study, published in the British Journal of Cancer, found that combating the effects of menopause with HRT could mean women are 2.7 times more likely to develop breast cancer than those who are not.
Women taking combined HRT are more likely to develop the disease, with risk increasing the longer it is used, according to results.
The study found those taking oestrogen plus progestogen HRT were 2.74 times more likely to develop breast cancer.
And those who had been using the combined HRT for 15 or more years had a 3.27 times increased risk, the study found.
But once HRT-use ended the increased risk returned to normal levels, the authors stressed.
Those taking oestrogen-only HRT did not have an increased risk, they said.
The findings were drawn from thousands of women taking part in the Breast Cancer Now Generations Study - a major study following more than 100,000 women for 40 years to investigate the causes of breast cancer.
Experts analysed data over six years from almost 40,000 menopausal women - 775 of whom developed breast cancer.
HRT is used to treat symptoms of menopause - such as hot flushes, migraines, disrupted sleep, mood changes and depression - by replacing hormones lost through menopause.
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