The length of time women were on treatment, varied across the trials from seven months to just over 10 years.
Overall, the results showed no evidence that hormone therapy provides any protective effects against death from any cause, and specifically death from cardiovascular disease, non-fatal heart attacks or angina, either in healthy women or women with pre-existing heart disease. Instead the findings showed a small increased risk of stroke for post-menopausal women.
The authors also explored how much of an effect there was of starting HRT earlier. They found some evidence that women who started treatment within the first 10 years of their menopause, when menopausal symptoms are most common, seem to have a small protection against death and heart attacks, and no increased risk of stroke. But even in this group, the risk of deep vein thrombosis (DVT) increased.
Author, Dr Henry Boardman from the Department of Cardiovascular Medicine at the University of Oxford, says the harms and benefits of hormone therapy vary according to the age of the woman, when they started their treatment.
He says, "The evidence we have provides some support for the so-called 'Timing Hypothesis', but we should bear in mind the size of this effect. When we looked at the results according to the age of women, or by how long since their menopause that they started treatment, we found that if 1000 women under 60 years old started hormone therapy we would expect six fewer deaths, eight fewer cases of heart disease and five extra blood clots over about seven years, compared to a 1000 similar women who did not start hormone therapy."
Dr Boardman continues "the findings of this Cochrane review need to be carefully considered. This is a complicated health issue, where the same treatment offers benefits in some women, but harms in others."
Dr David Tovey, Editor in Chief of the Cochrane Library commented, "This review adds a few more pieces to a complicated jigsaw of evidence relating to the use of HRT to treat symptoms of the menopause. The main analysis that the authors did, found no benefit and so we need to apply caution to the results from the subgroup analysis. However if true, this apparent benefit in preventing heart disease in younger women should be considered alongside other possible benefits and emerging evidence of harms, including the risk of breast cancer, ovarian cancer, and DVT."
The Cochrane Review's author, Dr Henry Boardman, concludes, "Hormone therapy remains a valid treatment option for women who are significantly troubled by menopausal symptoms, however the risks and benefits of such treatment vary according to age and medical history. Discussion with your GP is recommended when considering treatment."