A UK study has found that women who take combination hormone replacement therapy (HRT) are nearly three times as like to develop breast cancer.

Findings of the prospective study, led by researchers at The Institute of Cancer Research and published in the British Journal of Cancer, suggest that the increased risk of breast cancer from HRT is likely to have been underestimated by a number of previous studies.

The Breast Cancer Now Generations Study followed more than 100,000 women for 40 years to investigate the causes of breast cancer. Around 39,000 women with a known age at menopause were identified and monitored for six years, and given follow-up questionnaires to gather comprehensive data on any HRT use as well as their general health and lifestyle.

It was found that 775 of these women developed breast cancer, and that those using combined HRT (progestogen and oestrogen, for a median duration of 5.4 years) were 2.7 times more likely to develop the disease during the period of HRT use than those who had never taken HRT.

The risk increased with duration of use, with women who had used combined HRT for over 15 years being 3.3 times more likely to develop breast cancer than non-users. But the level of risk returned to normal after HRT use ends, the scientists found. Also of note, the researcher showed that women using oestrogen-only HRT were not at greater risk of developing breast cancer than those who had never used HRT.

As the findings were based on data adjusted for actual age at menopause and updated information on HRT use, the study authors believe they provide a more realistic estimate of the increased risk of breast cancer, and caution that several previously published studies are likely to have underestimated this risk.

"Whether to use HRT is an entirely personal choice, which is why it's so important that women fully understand the risks and benefits and discuss them with their GP. We hope these findings will help anyone considering the treatment to make an even more informed decision," commented Baroness Delyth Morgan, chief executive of Breast Cancer Now.

"On balance, some women will feel HRT to be a necessity. But in order to minimise the risk of breast cancer during treatment, it is recommended that the lowest effective dose is used for the shortest possible time".