Should you be worried about HRT tablets causing blood clots?

Woman taking HRT tablets

11 January 2019

Headlines have warned menopausal women of the risk of deadly blood clots from taking hormone replacement therapy tablets. We look behind the headlines. 

The research 

A study published in the British Medical Journal has found that women taking hormone replacement therapy (HRT) tablets have a higher risk of developing serious blood clots. Previous studies have found blood clots to be a rare but serious risk associated with HRT, but have not always been able to show how the risk varies between types of HRT.  

Scientists from the University of Nottingham compared the GP prescription records of 80,000 women aged 40-79 who had developed blood clots (such as deep vein thrombosis) and 390,000 women who had not. A deep vein thrombosis is a blood clot in one of the deep veins in the leg, which has the potential to dislodge and travel to the lung, where it can cause a fatal blockage.

The researchers focussed on prescriptions in the 90 day period before blood clots occurred, because previous research has shown that recent HRT use – rather than use in the past - is associated with increased risk of blood clots.  

They found that women who had taken HRT tablets were 58 per cent more likely to develop a blood clot within 90 days than those who hadn’t taken HRT. To put this into context, each year about 16 women in every 10,000 who had not taken HRT developed blood clots. In those who had taken HRT tablets, there were an extra nine cases per 10,000.  

They found that women who had taken HRT tablets were 58 per cent more likely to develop a blood clot within 90 days...

The increase in risk was higher for combined HRT tablets than for oestrogen only, and women taking equine oestrogen had a 15 per cent higher risk of developing blood clots than those taking treatments containing synthetic oestrogen (estradiol).  

The good news was that women who had used HRT absorbed through the skin, in the form of gels, patches or creams, did not have an increased risk of blood clots. However the scientists highlighted that these treatments were underused, with around 80 per cent of women opting for oral treatments instead. 

The expert view 

Professor Helen Stokes-Lampard, Chair of the Royal College of GPs, said: "For some specific symptoms, such as hot flushes and night sweats, HRT is the only medical treatment that has good evidence of benefit. 

"While this study is certainly interesting and important, as the authors themselves acknowledge, the findings do not prove that tablets cause more DVTs than patches, just that there is an association. As such, it is essential that more research is conducted in this area and taken into account as new clinical guidelines are updated and developed. 

"Current best practice is to prescribe the lowest possible dose of HRT for the shortest possible time, and so specific products and formulations of HRT are only initiated after a comprehensive discussion between the GP and their patient, and are tailored to meet the best interests of that individual. 

"It's important that patients don't panic or stop taking HRT as a result of reading about this study, but instead discuss their concerns at their next routine GP appointment, or seek advice from a reputable website like NHS Choices." 

How the research was reported 

Most news headlines focussed on the doubling of a deadly risk, but failed to put this into the context of the risk being very low to start with. The Daily Mail led with “Taking HRT pills to cope with the menopause 'DOUBLES the risk of suffering dangerous blood clots”. None of the news reports stated the risk of blood clots for women aged 40 to 79 who do not take HRT, which the study found to be 16 cases per 10,000 women per year.  

For the majority of women taking HRT, their risk did not double. The doubled risk was found with a particular type of combined HRT tablet (equine oestrogen with medroxyprogesterone acetate) which was taken by a relatively small proportion of women in the study. Of the 5795 women who had taken HRT and developed a blood clot, 740 of them were taking this type. 

For the majority of women taking HRT, their risk did not double.

The coverage also gave the impression that the increase in risk with HRT tablets compared with skin patches was a new revelation, whereas this has been demonstrated in studies previously, and is explained in the NHS guide to help women to decide whether to use HRT. 

The New York Times took a more positive view of the results, with the headline “Hormone Replacement Skin Patches Don’t Raise Risk for Blood Clots”, and The Telegraph highlighted that women could choose to switch to treatments with a lower risk: “Older menopausal women taking hormone replacement therapy (HRT) should be offered patches before pills”. 

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