Helen Johnson examines what the latest research from the US tells us about the risks of taking HRT

The department of health, the committee on Safety of Medicines (CSM) and doctors are urging UK women not to be alarmed by the recent announcement that a large-scale US trial of combination hormone replacement therapy (HRT) has been halted three years early after it was shown to increase the risk of invasive breast cancer by 26 per cent.

The US trial was designed to assess the risks and benefits of HRT intervention and, in particular, to address whether or not long-term use of oestrogen-progestogen combination HRT had a beneficial effect on the incidence of coronary heart disease in postmenopausal women. The study, part of the Women's Health Initiative (WHI) series of clinical trials and observational studies, involved more than 16,000 postmenopausal women, aged 50-79 years, who had not previously had a hysterectomy.

The trial found that women taking this particular form of combination HRT had a 26 per cent increased risk of developing invasive breast cancer, a 29 per cent increased risk of heart attack and a 41 per cent increased risk of having a stroke. Conversely, the risks of developing colorectal (bowel) cancer and osteoporotic bone fractures were significantly reduced.

So what do the latest research results mean for the six million women in the UK taking some form of HRT?

Whilst it is undoubtedly true that these findings represent an important addition to the existing body of knowledge about the relative risks and benefits of HRT, women should not be unduly alarmed by them. Doctors have long been aware of the association between HRT and breast cancer and have been quick to point out that the small increase in the number of breast cancer cases witnessed during the WHI trial merely confirms previous studies. Even then experts are stressing that the absolute risks to women remain very small. According to the WHI findings, 38 out of every 10,000 women taking HRT in any one year can be expected to develop breast cancer, compared to 30 cases where HRT is not being used, only eight more cases per 10,000 women per year.

The WHI cardiovascular disease findings are somewhat more surprising, but the BMJ stresses that other forms of HRT, not the subject of the WHI study, may still confer a beneficial effect on cardiovascular risk. Although all HRT regimens include oestrogen, there are as many as 50 different regimens available in a variety of tablets, nasal sprays, implants, skin patches and vaginal rings. Not all HRT regimens are the same; different types of oestrogen and progestogen have different metabolic effects on the body and the route of administration can impact the level of absorption.

It is important that women in the UK understand the limitations of the WHI study design and its findings. The trial looked at only one particular form of combination HRT which is not used either in the UK or Europe. There is little to suggest that the WHI conclusions would be replicated in other HRT regimens or even in lower doses of the same combination.

The trial evaluated the effects of taking HRT continuously and in the long-term. It did not consider the short term usage (less than 5-10 years) of HRT for the relief of menopausal symptoms. Current medical opinion suggests that short term HRT use is unlikely to increase risk appreciably. In fact, the health and quality of life benefits of short term use may outweigh the risks for many women.

So what should UK women taking HRT do next?

Firstly it is important that women assess the relative health risks and benefits according to their particular individual needs and risk factors. For a woman at high risk of developing osteoporosis, for example, the benefits of taking HRT for several years may well outweigh any increased breast cancer risk. Depending upon the type of HRT regimen used, there may still be significant health benefits to taking it - a reduced incidence of osteoporotic fractures, colorectal and other forms of cancers, as well as relief from distressing and debilitating menopausal symptoms. Just weeks after publication of the WHI findings, researchers at the University of Sheffield and John Radcliffe Hospital in Oxford concluded that long term continuous use of combination HRT (a different combination from that studied in the WHI trial) can help protect against endometrial cancer (cancer of the lining of the womb).

Women need clear information and guidance to enable them to weigh up the relative risks and benefits and make informed choices according to their individual needs, circumstances and risk factors. Healthcare professionals, health development agencies and the government all have a role to play here. Whilst government agencies were quick to issue guidance on the latest US findings, their response, particularly regarding the likely cardiovascular risk, has been criticised as unhelpful.

It is evident that further clinical research is required, particularly into the long term effects of taking HRT continuously. The Medical Research Council has already recruited 5,000 UK women to participate in the WISDOM trial, a study designed to answer important questions about the long term risk and benefits to taking HRT. Perhaps when this trial ends in 2012, the picture will be clearer for the millions of women trying to determine whether taking HRT really is worth the risk.