NHS Choices has conducted an assessment of press reports that suggest hormone replacement therapy (HRT) does not increase the risk of breast cancer and the study that reported the link in 2002 was “fundamentally flawed.”

The reports are based on a new analysis of the data from the following three studies that aimed to determine whether the association between HRT and breast cancer actually showed that HRT caused an increased risk in breast cancer:

• The Collaborative Reanalysis, published in 1997, which had pooled data from 51 studies that looked at the risk of breast cancer in relation to HRT use.

• The Women’s Health Initiative, which included two RCTs published in 2002, the first involved approximately 5000 women who received oestrogen-only HRT and 5000 who received a placebo for an average of 6.8 years; the second study involved more than 8000 women who received oestrogen plus progesterone HRT and more than 8000 women who received a placebo; they were followed for an average of 5.2 years.

• The Million Women Study published in 2003- a prospective cohort study that followed more than a million UK women aged over 50 years.

Each study was appraised against nine criteria that would need to be met before it was possible to say whether HRT caused breast cancer. The new analysis did not suggest that the original studies were poorly performed but it did highlight limitations to what the studies can tell us. The analysis found that all three studies failed to meet the majority of the criteria and therefore none of the studies was able to establish whether HRT increases the risk of breast cancer. These findings were published in the Journal of Family Planning and Reproductive Health Care.

The assessment concludes that “this research is important because it makes us reconsider the evidence that suggested HRT might increase risk of breast cancer rather than there just being an association between the two. This new research shows that these studies can only tell us that there may or may not be a link between HRT and increased risk of breast cancer. Further, carefully designed studies are needed to determine whether HRT does indeed cause an increased risk of breast cancer.” It advises women who are taking HRT for menopausal symptoms or who used to take HRT should note the advice from Cancer Help UK, which states that any cancer risk associated with HRT reduces to a normal level five years after stop treatment.

A leading epidemiologist has condemned the authors for writing and the journal for publishing the articles challenging this well established link, describing their actions as “a disgrace” and their arguments “tortuous and ridiculous.” He told the BMJ, “Thirty pages of journal articles dedicated to questioning an established causal relationship between HRT and breast cancer requires an explanation. It is not a tenuous relationship, as we know from the Women’s Health Initiative randomised trial and the concurrent observation of reduction in oestrogen receptor positive breast tumours in populations at the time of dramatic reduction in the use of HRT. Why then do we need to study these papers to find out why we have it so wrong?” The editor of the journal said: “The findings from these studies have been elevated from association to a causal relationship, which, as demonstrated in these articles, is not justified. It is a basic principle of epidemiology that association does not prove causation…. This is especially important because it can equally validly be argued that considerable damage has been done to women’s health through misunderstandings such as the above, because doctors are now reluctant to prescribe HRT, even to women who are clearly in considerable distress due to their symptoms…..These factors alone justify reasoned debate. The matter should definitely not be considered proven or closed despite what some say.”