The long-term safety of oestrogen-only hormone replacement therapy does seem to increase the risk of breast cancer, but only after more than 10 years' use, according to the latest study to investigate the matter.

Part of the large-scale Nurse’s Health Study, the trial enrolled nearly 29,000 post-menopausal women who had had a hysterectomy and looked at the effects of oestrogen-only HRT on breast cancer risk over an extended follow-up period.

The results suggested that users of so-called unopposed oestrogen – in other words without the addition of a progestin - were at increased risk of breast cancer, but only after 15 years of use. Researchers said the findings should be reassuring for women who want to use oestrogen for a short time to relieve menopausal symptoms, although those using it for longer may want to discuss this with their physician, they suggested.

That said, for women who had been on oestrogen for at least 15 years, the risk of hormonally driven breast cancer climbed 48%, while at 20 years, the risk of any type of breast cancer rose 42%. The risk of breast cancer also appeared to rise between 10 and 15 years of use, but the increase was not statistically significant, the researchers said.

These results back up new findings from the Women’s Health Initiative study, reported last month, which also found no increase in breast cancer risk with unopposed oestrogen among post-menopausal, hysterectomised women who had received HRT for seven years. As with the WHI study, NHS found that there was a trend towards a decrease in the risk of breast cancer in the early years of oestrogen-only HRT use.

Both studies are a boost to Wyeth, which manufactures Premarin, one of the leading oestrogen-only HRT brands. Although other studies of HRT have indicated an increased risk of breast cancer – including the WHI itself three years ago – these have tended to include women on oestrogen and progestin combinations, such as Wyeth’s own PremPro product.

The new data on breast cancer safety – as well as results suggesting that HRT can have a cardioprotective effect in younger women, seems to be benefiting sales of Premarin (conjugated oestrogens), which saw its turnover advance 26% to $266 million in the first quarter of this year.

"The NHS findings, in addition to recent WHI study findings on breast cancer, are very important for millions of women who are appropriate candidates for oestrogen-alone therapy," commented Ginger Constantine, vice president, women's health care and bone repair, for Wyeth Pharmaceuticals.

Oestrogen/progestin combinations are still used for younger women who have not undergone a hysterectomy and so could be at risk of an increased risk of uterine cancer with unopposed oestrogen.

The findings of the NHS are published in the May 8 issue of the journal Archives of Internal Medicine.