New UK research has found a link between hormone replacement therapy (HRT) tablets and a higher risk of rare but serious blood clots known as venous thromboembolism or VTE. However, there appears to be no increased risk when using HRT skin patches, gels and creams.
Carried out by researchers at the University of Nottingham, the new large-scale study looked at the HRT prescription records of 80,396 women aged 40 to 79 years who developed blood clots and compared them to those of 391 494 women who did not.
The researchers also took into account other potentially influencing factors, such as lifestyle, family history of blood clots and any underlying conditions linked to blood clots.
The findings, published in The BMJ, found that using HRT tablets was associated with a significantly increased risk of VTE risk (9 extra cases per 10,000 women per year) compared with not using HRT.
Moreover, HRT tablets containing equine estrogen, including single and combined tablets, were consistently associated with higher risks than tablets containing synthetic oestrogen.
Higher doses of oestrogen were also associated with a higher VTE risk.
No increased VTE risk was found for skin patches, gels and creams, however despite this the majority of women choosing HRT are prescribed oral treatments.
HRT is used to relieve menopausal symptoms such as hot flashes and night sweats, with different treatments available depending on the symptoms, for example tablets containing estrogen only or a combination of estrogen and progestogen, as well as patches, gels and creams.
Previous studies have also shown that menopausal women using HRT have an increased risk of blood clots, but there has been a lack of information on the risks associated with different types of HRT.
As an observational study the researchers note that they cannot establish cause and effect, and some limitations to the study may have affected the results. However, they do add that the study “has provided a more detailed picture of the VTE risks for different HRT preparations and can help clinicians and women make treatment choices.”