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Can HRT cause cancer?

Can HRT cause cancer?

Okay, let’s talk numbers at the top. HRT, on average, increases the risk of breast cancer by around four extra cases per 1,000 women after five years, and ovarian cancer by one extra case per 1,000 users in the same timeframe.

So in a nutshell: yes, but the risk is low. And there’s a bit more to it than that. Studies show that using HRT can reduce your risk of heart disease and osteoporosis, as well as help maintain muscle strength.

Alexandra Cristina Cowell
Medically reviewed by
Alexandra Cristina Cowell, Writer & Clinical Content Reviewer

Medicine tends to be a balancing act, and using HRT for menopause symptoms can tip the scales in your favour.

Put simply, the benefits quite often outweigh the risks.

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Medically reviewed by
Dr Alexandra Cristina Cowell
Writer & Clinical Content Reviewer
on January 15, 2024.
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Alexandra Cristina
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HRT: A brief overview

HRT (hormone replacement therapy) is a treatment for symptoms of perimenopause – the transitional period where you eventually stop getting periods. During this time, your body stops producing the hormone oestrogen, and for many women this can cause symptoms like hot flushes and brain fog, as well as increase the risk of long-term health problems like osteoporosis. To counter these symptoms, HRT simply works by increasing your oestrogen levels, negating the ‘deficit’ caused by menopause.

The history of HRT and its link to cancer

Conjugated oestrogen for menopause symptoms, under the brand name Premarin, was first licensed in 1941 in Canada, 1942 in the US, and 1956 in the United Kingdom. However it wasn’t until the 1960s that its use became more widespread — largely thanks to the book Feminine Forever, which championed synthetic oestrogen as a way for women to ‘maintain their youth and femininity.’ In the years following the publication of this book, HRT sales quadrupled.

However, in 1975 it was found that taking oestrogen on its own (unopposed) increased the risk of endometrial cancer, leading to a huge drop-off in its use. This may well have been the end for HRT, had it not been discovered shortly after that taking a progestogen alongside a lower dose of oestrogen dramatically lowered this risk. And so, ‘combined HRT’ was made available for women who hadn’t had a hysterectomy.

HRT was popular again. And continued to be for the rest of the 20th century — becoming one of the most prescribed drugs in the US by the 1990s. An increasing body of research continued to find new benefits of HRT, showing that it lowered the risk of heart disease and osteoporosis. However this all changed (again) at the beginning of the 2000s.

The two studies and the health scare

Two studies: one ‘split study’ from the US called the Women’s Health Initiative (WHI), which looked at the effects of both combined and oestrogen-only HRT, and one from the UK called The Million Women Study (MWS), changed perceptions of HRT for decades to come. The WHI studies ran from 1993 to 2002/2004 (the study on combined HRT was stopped two years earlier than the oestrogen-only one), and the MWS ran from 1996 to 2001.

Both studies seemed to find increased health risks from HRT at early stages, including slight associations with breast cancer, heart disease, stroke and blood clots. It now seemed that the risks of HRT outweighed the benefits. These preliminary findings were published and widely circulated, leading to a lot of negative press, fear and ‘health scare’ headlines.

Following this, doctors stopped prescribing it (or became more wary of doing so), and women stopped taking it. The result of which can still be seen today — HRT use declined by a dramatic 46% in the five months following the early pause of the WHI trial in the USA. And following the results of the WMS, the number of postmenopausal women in the UK using HRT dropped from 29% in 2001/02, to less than 11% by 2005.

The oestrogen-only arm of the WHI trial continued until 2004, when its preliminary findings were also published. Like the combined HRT trial, it stopped prematurely – this time because of a small increased risk of stroke, despite finding significant other benefits (like a reduction in cardiovascular disease and osteoporosis) and no increased risk of breast cancer. The overall message surrounding HRT remained negative.

In the years since these trials, their limitations have become increasingly apparent, and new evidence has been found that goes against much of the original findings. The WHI trial, for example, mainly involved women who had their last period more than a decade before, at a dose we now know to be too high for older women. It also only tested just one form of either HRT. Basically, it didn’t account for how well the right dose and form of HRT could benefit women in order to outweigh or reduce the risks.

There was an issue with the WMS too: the women in the trial were ‘self-selecting’ (they were surveyed during a breast screening) rather than randomised, which often skews results. It could be, for example, that the women attending their breast screening had already found a lump, meaning that those enrolling in the trial were already more likely to have breast cancer than the general population.

But despite growing knowledge within the scientific community that the risks of HRT found by these studies did not reflect the general population, the reputation stuck for much of the world.

So where are we up to now?

Well, we now know that the risk of breast cancer from combined HRT originally reported by these trials is much lower than previously thought. The risk is now understood to be equivalent to around four extra cases per 1000 women after five years (less than smoking, alcohol and obesity). So there is a risk, but it’s small. And what’s more, the science shows that, broadly speaking, the benefits of HRT outweigh the risks, especially for symptomatic women under the age of 60, or those within ten years of menopause.

But despite new information on the topic, HRT scepticism remains throughout many countries. Britain seems to be a bit of an outlier here, with HRT prescriptions increasing by 35% from 2021 to 2022, but for much of the world it can be argued that HRT use remains low.

How does HRT increase the risk of breast cancer?

It’s not fully understood how HRT increases the risk of breast cancer, but it’s thought to be due to increased levels of the hormone oestrogen. That’s because certain types of breast cancer use oestrogen to ‘fuel’ their growth, so increased levels of oestrogen can help the cancer cells to grow and spread.

When is the risk higher?

Evidence shows that combined HRT carries a higher risk of breast cancer than oestrogen-only, and that the longer you take HRT for, the greater the risk. Dosage can also play a role (bigger dose tends to equal bigger risk). That’s why it’s recommended that you only use HRT for as long as you have symptoms, at the lowest dose that helps them where possible. Once you stop using HRT, this increased risk goes down over time.

Your risk is also increased if:

  • you’ve had breast cancer before, or if you have a family history of it;
  • you’re over the age of 60;
  • it’s been more than ten years since the onset of menopause;
  • you take HRT for more than five years.

Can HRT cause ovarian or any other kind of cancer?

Alongside breast cancer, HRT has been linked to two other kinds of cancer: ovarian and endometrial (womb). Continuous combined HRT increases the risk of ovarian cancer slightly, and doesn’t affect womb cancer risk at all. Oestrogen-only HRT also carries a slight increase in ovarian cancer risk, but a notably increased risk of womb cancer. This is why oestrogen-only HRT is only prescribed for women who have no risk of womb cancer (usually because they’ve had a hysterectomy). Again, these increased risks go down over time once HRT is stopped.

Does cancer risk vary by HRT type?

Yes, as well as varying risks between combined or oestrogen-only HRT, the delivery (patch, gel, tablet) and form of hormone can also make a difference. Norethisterone is linked to the highest risk of breast cancer, and dydrogesterone the lowest. Medroxyprogesterone and levonorgestrel were also associated with increased risk, according to recent research. It’s also thought that transdermal HRT (delivered through the skin by gel, patch or spray) is safer than oral (taken by tablet).

Are there any risk-free alternatives?

If you’re looking for help with menopause symptoms but are concerned about the risks of HRT, there are safer options for you.

Tibolone is a new form of treatment that offers an alternative to traditional HRT, as it’s hormone-free. It’s not thought to be as effective though, and it’s rarely prescribed. Local HRT, such as vaginal gels and pessaries, can also be beneficial if you mainly struggle with vaginal symptoms rather than ‘full body’ ones, like hot flushes. Because the oestrogen in local HRT only works in your vagina, it doesn’t carry the same risks as systemic (whole body) HRT.

And lastly, it’s important not to forget the importance of healthy lifestyle choices when it comes to menopause health. A good diet, regular exercise and stress management can all play a vital role in improving overall health and reducing symptoms, which can reduce your need for high doses of HRT.

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When we present you with stats, data, opinion or a consensus, we’ll tell you where this came from. And we’ll only present data as clinically reliable if it’s come from a reputable source, such as a state or government-funded health body, a peer-reviewed medical journal, or a recognised analytics or data body. Read more in our editorial policy.

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