Hormonal replacement therapy for the menopause increases the risk of breast cancer and that risk can persist for more than a decade after usage stops.
Researchers from the University of Oxford analysed data from more than 100,000 women with breast cancer from 58 epidemiological studies worldwide.
They concluded that Menopausal hormone therapy (MHT) was responsible for around one million breast cancers in western countries – one-twentieth of the total since 1990.
The study, published in The Lancet also revealed that even after stopping use, the excess risk of breast cancer was found to persist for more than 10 years, with the size of the risk linked to the duration of previous use.
All types of MHT except topical vaginal oestrogens were linked to an increased risk of breast cancer.
Researchers found that the risks were greater for users of oestrogen-progestagen hormone therapy than for oestrogen-only hormone therapy.
Women typically begin MHT at around the time of the menopause, when ovarian function ceases, causing symptoms including hot flushes and discomfort.
Around 12 million women use MHT in Western countries, with approximately six million users in North America, six million in Europe, including one million in the UK.
Five years of MHT use is now common, though regulatory bodies in Europe and the USA recommend it is used for the shortest time that is needed.
Co-author Professor Valerie Beral from the Nuffield Department of Population Health, University of Oxford, said: “Before all we knew was that the risk was increased when they used, the belief was that it went away when they stopped.
“The main finding is that we now know the long-term effects, that the risk persists for more than a decade after stopping.”
Researchers also found that the risk was twice as great for women who used MHT for 10 years. However there was little excess risk after using any form of MHT for less than a year.
Prof Stephen Evans, Professor of Pharmacoepidemiology, London School of Hygiene and Tropical Medicine (LSHTM), said these results should “not cause alarm among women”.
He added: “But there is no doubt that they should follow the advice given by the MHRA in the UK and the FDA in the US that MHT be used for the shortest time that it is needed.
“MHT does offer real benefits for menopausal symptoms, but its use beyond one year seems to confer a steadily increasing risk of breast cancer with increasing years of use.”
Dr Julie Sharp, head of health and patient information at Cancer Research UK, which funded the study, said: “HRT is an effective treatment for menopause symptoms, and we’ve known for some time that it raises the risk of breast cancer. But this study found the risk may persist for longer after stopping HRT than we previously thought, so women should think carefully about taking it.”