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Doubts raised over HRT cancer risk
Further doubt has been cast on a study suggesting hormone replacement therapy (HRT) increases the risk of breast cancer.
A link between HRT and the disease has proved controversial, with several studies suggesting the finding has been blown out of proportion.
However, other research has reinforced the link or shown a drop in breast cancer rates alongside declining HRT use.
Writing in the Journal of Family Planning and Reproductive Health Care, experts said the Million Women Study in the UK - which has reiterated the link several times - does not in fact establish HRT as a cause of breast cancer.
However, they did not rule out that HRT may increase risk of the disease.
First published in 2003 and funded by Cancer Research UK, the Million Women Study found that using combined HRT doubles a woman's risk of developing breast cancer compared with women not taking HRT.
An update in August last year found similar results after a longer follow-up, including a "rapid fall in risk after HRT is stopped".
But experts from South Africa, Germany and the UK analysed the study and said: "HRT may or may not increase the risk of breast cancer, but the Million Women Study did not establish that it does."
The authors examined criteria applied to scientific research to show a causal link, such as biases and biological implausibility, to review the findings of the study.
Their analysis highlighted several design flaws that they say would have skewed the findings. For example, cancers detected within a few months of the study's start would have already been present when the women were enrolled in the research, but these were not excluded, they said.