The risk of breast cancer associated with hormone replacement therapy in postmenopausal women is greater than previously thought and is seen after just a year of use, according to a new study.

Published in The Lancet, the analysis of combined data from 58 studies involving more than 108,000 postmenopausal women with invasive breast  cancer found an absolute increase of 2% in risk from 6·3% to 8·3%, or one extra cancer for every 50 women who used combined HRT for five years starting at the age of 50.

The increased risk was not so great for oestrogen-only hormone preparations, rising from 6·3% to 6·8%, equivalent to one additional cancer in every 200 users over five years.

And in contrast to previous studies that suggested an increased breast cancer risk becoming evident after five years of HRT use, the excess risks were seen with one to four years of use, and progressively greater risks with longer use.

The excess risk among users of HRT was greater for ER+ than for ER– disease, did not differ substantially between the main oestrogenic constituents, or by whether oestrogens were administered orally or transdermally.

Writing in the Conversation, Professor Martha Hickey, Professor of Obstetrics and Gynaecology, University of Melbourne said the the new information raised questions about international guidelines that suggest healthy women around the average age of menopause can safety take menopausal hormone therapy for up to ten years.

“Another important finding was the persistent increased risk of breast cancer for up to ten years after stopping menopausal hormone therapy. This is considerably longer than was previously thought,” she wrote in an article co-authored by Dr  Mark Jenkins, Director of the Melbourne University Centre for Epidemiology and Biostatistics.

“For women going through menopause at the average age, menopausal hormone therapy should only be used to treat troublesome menopausal symptoms, not for the prevention or treatment of other conditions,” they recommended.

However the Australian Menopause Society said in a statement that the study findings could not be extrapolated to current HRT prescribing practices, because the cancer data was based on formulations and doses non longer used to treat postmenopausal women.

“Specifically, the use of the progestogens medroxyprogesterone acetate and norethisterone (norethindrone) is now discouraged because of their known adverse effects, but these account for nearly all of the data for combined estrogen-progestogen therapy included in the paper,” it said

The AMS statement also drew attention to the study finding that HRT had little effect on cancer risk in obese women obesity. The study authors said this might be because the amount of oestradiol from endogenous production in the adipose tissue is greater than that contributed from use of HRT, and endogenous hormone concentrations are higher.

But the AMS said this showed the  that obesity is an important risk factor for breast cancer.

“Considering the high proportion of women over 50 who are overweight or obese, and the progressive increase in we are seeing in the prevalence of obesity, this is an important public health message.”