Oestrogen-only replacement therapy is not associated with an increased risk of breast cancer in post-menopausal women, and may even give some protection against the disease, according to the latest findings from the large-scale Women’s Health Initiative study.

The results contrast starkly with an earlier analysis from the WHI, which reported three years ago that postmenopausal women given both oestrogen and progestin showed a definite spike in breast cancer risk.

The data provide a boost for Wyeth, which makes the conjugated oestrogen product, Premarin, used in the WHI study. Sales of Premarin and a related oestrogen/progestin product PremPro have been hit hard by the earlier WHI results, with a knock-on effect for other hormone replacement products aimed at post-menopausal women.

“Findings about breast cancer risk and hormone use have been mixed. Some studies have reported an increased risk of breast cancer with hormone therapy. Other studies have not shown this increase,” said Wyeth in a statement.

Now, the latest results suggest that in women who have had a hysterectomy - and so are protected from the well-documented increase in uterine cancer with oestrogen-only replacement therapy - could benefit from treatment for post-menopausal symptoms without hiking their risk of breast cancer.

"We know there are many symptomatic menopausal women who are appropriate candidates for oestrogen therapy but are afraid to take it due to concerns about breast cancer risk," commented Ginger Constantine, vice president, women's health care and bone repair at Wyeth Pharmaceuticals.

Overall, the study authors conclude that postmenopausal women who have had a hysterectomy and were treated with conjugated oestrogens alone for an average of 7.1 years did not have an increased incidence of breast cancer. Further analyses found that women who consistently took oestrogen as prescribed had a statistically significant decrease in breast cancer risk compared to women taking placebo.

However, the results run counter to findings from another study reported this week.

The Black Women's Health Study, a questionnaire-based study of 32,559 women age 40 years or older, found an increase in breast cancer risk with hormone replacement, although the conclusions are complicated by the broad range of products used, including both oestrogen-only and combination therapy.

One possibility is that observational studies, which can monitor patients over decades, can pick up longer-term effects of treatment that do not emerge in relatively short trials such as the WHI.

These results are published in the Archives of Internal Medicine, alongside an article detailing the risks of venous thrombosis (VT) with oestrogen-alone therapy, which were seen in both portions of the WHI study. These risks are already documented on labelling of oestrogen-replacement products, including Premarin.

The results of the WHI study are published in the April 12 issue of the Journal of the American Medical Association.