The increased risk of breast cancer from menopausal hormone therapy lasts more than a decade after treatment stops, a major report suggests.
The researchers at the University of Oxford say the results mean the risk of breast cancer is double what women are currently being told.
They estimate a million cases of breast cancer may have been caused by the therapy in the West since the 1990s.
Charities said the drugs should be used for the shortest time necessary.
What is menopausal hormone therapy?
The menopause is when women's periods stop and they can no longer become pregnant naturally.
In the UK, the average age to reach the menopause is 51.
There is a marked fall in two hormones in the body – oestrogen and progesterone – which can have profound effects throughout the body.
It can result in hot flushes, night sweats, mood swings, vaginal dryness and reduced sex drive.
Menopausal hormone therapy (MHT) essentially replaces the missing hormones to alleviate menopausal symptoms.
It is commonly referred to as HRT (hormone replacement therapy), although this also includes other therapies such as thyroid hormones, growth hormones or testosterone which are not part of this study.
MHT comes in different forms such as tablets, gels, patches or vaginal creams. It can be made up of different hormones – some are just oestrogen, others include progestagen (synthetic hormones that simulate progesterone).
However, all drugs have side-effects and in this case that includes an increased risk of cancer.
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How big is the risk?
The study calculated six in every 100 women not taking menopausal hormone therapy would develop breast cancer between the ages of 50 and 69.
If they took oestrogen and progestagen every day for five years, eight of the women would develop breast cancer.
So out of every 50 people taking the combined therapy, one would develop breast cancer as a result of the drugs.
There are other types of hormone replacement therapy and each of those showed an increased risk too.
Taking intermittent hormone therapy (daily oestrogen, but progestagen for around half the monthly cycle) led to one extra case of breast cancer in every 70 people.
And just taking oestrogen caused an extra case in every 200 women.
However, taking oestrogen alone increases the risk of womb cancer and is normally used only after a hysterectomy.
How much of this is new?
The risk of breast cancer from menopausal hormone therapy is already well known.
It is in the official guidance on the use of these drugs and does form part of the conversation between patient and doctor when balancing the risks and benefits.
What is new is showing the increased risk of breast cancer lasts more than a decade and the impact that has.
"Previous estimates of risks associated with use of menopausal hormone therapy are approximately doubled by the inclusion of the persistent risk after use of the hormones ceases," said Prof Valerie Beral from the University of Oxford.
She also said the study showed menopausal hormone therapy at any age increased breast cancer risk.
"There was a myth going round if started before age 50 there was no risk, but that's really not true. This is a myth you'll find really quite widespread," Prof Beral said.
Does it matter how I take menopausal hormone therapy?
Yes.
The study, published in the Lancet, showed nearly all types of therapy (pills, gels, patches) increased the risk of breast cancer.
Oestrogen therapies applied in the vagina – such as creams or a pessary – did not increase the risk.
With these methods the therapeutic hormones do not reach the bloodstream and circulate around the body.
It means they do not provide all of the benefits of menopausal hormone therapy, but do not have all the risks either.
What should people do?
Deciding whether to use menopausal hormone therapy involved balancing the risks and the benefits before this report was published and the same is true now.
"We don't want to alarm women, but we don't want to give them false reassurance," Prof Gillian Reeves, from the University of Oxford told the BBC.
She added: "What we would hope is women use this information to make a much more informed decision about whether they want to start taking HRT or continue taking HRT."
The Royal College of GPs, in the UK, urged patients "not to panic" and continue with their prescription.
If patients are concerned, it recommends discussing their hormone therapy at their "next routine appointment".
Baroness Delyth Morgan, the chief executive at Breast Cancer Care and Breast Cancer Now, said: "On balance, many women will feel HRT to be a necessity, as it can be really effective in helping them control debilitating menopausal symptoms such as hot flushes.
"In order to minimise the risk of breast cancer, it is normally recommended that the lowest effective dose is used for the shortest possible time."
What do patients think?
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Louise Rivers, 51 from Bracknell in Berkshire, started having symptoms of the menopause three years ago.
She says she "lost her brain" and was struggling at work, her joints ached, she was not sleeping well and she starting getting migraines.
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