Hormone replacement therapy (HRT) is used by millions of women around the world to help relieve uncomfortable menopause symptoms.
While the controversial treatment may ease night sweats, hot flushes and vaginal dryness, it has been linked to an increased risk of breast cancer, the NHS reports.
New research suggests, however, the odds may vary depending on the specific hormones given.
A review of more than 26,000 women by the University of California, LA, (UCLA) found post-menopausal women given oestrogen alone were 23% less likely to develop breast cancer.
When oestrogen was combined with the drug progestin, which mimics progesterone, the risk went up by 29%.
The average age of menopause in the UK is 51, according to the NHS.
While unpleasant symptoms usually pass in a few years, some opt to take HRT. This replaces hormone levels that have reduced, like oestrogen and progesterone.
HRT comes in different forms, including tablets, skin patches and implants. All forms aside from vaginal oestrogen are known to raise the risk of breast cancer if taken for more than one year, according to the NHS.
If taken for five years, up to one “extra” case of the disease could develop in every 50 women on the treatment.
HRT aside, one in eight women in the UK and US will develop breast cancer at some point in their life, statistics show.
“After nearly half a century, menopausal hormone therapy influence on breast cancer incidence and mortality remains unsettled”, study author Dr Rowan Chlebowski said.
To learn more, the scientists compared the breast cancer risk of women who received either oestrogen and progestin, oestrogen alone or placebo for around five years in two separate trials.
They found the women on just oestrogen were less likely to be diagnosed with breast cancer and 44% less likely to die from the disease over the next 16 years compared to those on placebo.
The women on oestrogen and progestin, however, were more likely to be diagnosed with breast cancer over the next 18 years versus those on placebo.
Full results are being presented at the 2019 San Antonio Breast Cancer Symposium.
Exactly why this occurs is unclear. Higher levels of oestrogen may help some breast cancers grow, according to Breast Cancer Now.
Cancer Research UK acknowledges HRT can raise the risk of breast, ovarian and womb cancer.
It adds, however, the odds depend on the type of treatment, how long a woman is on the therapy and her underlying health.
Prevent Breast Cancer adds the risk of HRT needs to be balanced against its potential benefits.
One expert noted the HRT used in one of the two studies, which started in 1993, is no longer available in the UK.
“Modern forms of HRT may have different safety profiles, hopefully better, as modern HRT is more like the body's own hormones,” Dr Melanie Davies, from University College London Hospitals, said.
She added women on HRT should not come off it or change their regimen based on the results.
The UCLA scientists note the women were only given one dose of HRT, with different findings potentially applying to other schedules.