Taking certain types of hormone replacement therapy (HRT) can double the risk of developing breast cancer, says a study of more than a million women.
Breast cancer could be more deadly after HRT
The largest ever study into the link between HRT and breast cancer was conducted by scientists at Cancer Research UK's Epidemiology Unit in Oxford.
The research suggests the single pill moderately increases the risk of breast cancer, but the combined pill doubles the risk.
It estimates HRT, taken by women to relieve the unpleasant symptoms of menopause, may have been responsible for an extra 20,000 cases of the disease in Britain in the last decade.
The Committee on the Safety of Medicines has reviewed the data and written to all health professionals.
They stressed short-term HRT use is still beneficial, but those taking it for more than a year should heed the risks and discuss them with their GP.
Barbara Sims took the combined HRT for six years before being diagnosed with breast cancer.
But she said she had no regrets because it rescued her from an early menopause and may have helped the detection of the cancer.
"Because my symptoms were so debilitating I couldn't lead a normal life at all," she said.
About 1.5million women in the UK take HRT, with half taking the combined version.
Hundreds of women have contacted the helpline NHS Direct since hearing of the link with breast cancer.
The researchers estimate there have been 20,000 cases of breast cancer over the last decade in women aged 50 to 64 because of HRT.
They say combined HRT is responsible for 15,000 of those cases.
The study is also the first to report that HRT increases the risk of dying from breast cancer, by 22%.
About 20 women in every 1,000 will usually develop breast cancer.
But the study found for every 1,000 women who use HRT for 10 years from the age of 50, there will be an additional 19 cases of cancer in those using the combined oestrogen and progestogen version and an extra five in those using oestrogen-only HRT.
Using tibolone, a steroid treatment, also increased a woman's cancer risk.
Women also have to bear in mind that oestrogen-only HRT carries an increased risk of uterine cancer.
Women's risk of developing breast cancer decreases when she stops and is back to normal levels after five years, claims the research.
The data, published in The Lancet, covered a million women who went for mammograms between 1996 and 2001.
Professor Valerie Beral, who led the research, said: "Since our results show a substantially greater increase in breast cancer with combined HRT, women need to weigh the increased risk of breast cancer caused by the addition of progestogen against the lowered risk of uterine cancer."
What is HRT?
In late middle age, a woman has reduced levels of some sex hormones.
This causes unpleasant symptoms such as hot flushes, mood swings, loss of libido.
HRT aims to boost hormone levels to reduce these symptoms.
Dr John Toy, Medical Director of Cancer Research UK, said: "It would be sensible for a woman to take HRT for only as long as it is necessary to deal with her medical problems as advised by her doctor.
"A woman wanting to take HRT for a long time would be extremely wise first to consider carefully the findings of this large study and other relevant research."
Martin Ledwick, senior cancer information nurse for CancerBACUP, said: "We know from the calls we get to our helpline, that the impact of HRT on breast cancer risk has been of concern to women for some time, so it's good to have more evidence on this issue.
"This will help women to make more informed choices about their health, but it will also cause anxiety - particularly amongst women who have been on combined HRT for some years."