Hormone replacement therapy can increase the risk of more advanced, potentially deadly breast cancer, a study has shown.
Death rates from the disease were found to double in a group of women who underwent HRT for an average 5.6 years.
A link between HRT and breast cancer in post-menopausal women is well known, and was demonstrated by a large American study called the Women's Health Initiative (WHI).
Now new findings from the WHI show that use of oestrogen plus progestin hormone therapy increases the incidence of more advanced forms of breast cancer.
It is also associated with a higher risk of deaths attributed to the disease.
Women taking part in the study were either given combination HRT or a dummy "placebo" pill.
Almost 13,000 women consented to having their condition monitored for an extended 11-year period after treatment.
Invasive breast cancer occurred in 385 women who received hormone treatment compared with 293 who did not.
A significantly larger percentage of women in the HRT group had breast cancers that had spread to the lymph glands.
There were 25 deaths from breast cancer among the HRT group and 12 among the placebo group. At a population level, this was the equivalent of 2.6 against 1.3 deaths per 10,000 women per year.
The findings were reported in the latest issue of the Journal of the American Medical Association.
Rowan Chlebowski, from the Los Angeles Biomedical Research Institute, and colleagues wrote: "With some exceptions, the preponderance of observational studies have associated combined hormone therapy use with an increase in breast cancers that have favourable characteristics, lower stage, and longer survival compared with breast cancers diagnosed in non-users of hormone therapy.
"However, in the WHI randomised trial, combined hormone therapy increased breast cancer risk and interfered with breast cancer detection, leading to cancers being diagnosed at more advanced stages.
"Now, with longer follow-up results available, there remains a cumulative, statistically significant increase in breast cancers in the combined hormone therapy group, and the cancers more commonly had lymph node involvement. The observed adverse influence on breast cancer mortality of combined hormone therapy can reasonably be explained by the influence on breast cancer incidence and stage."Reuse content