Doubt has been cast on research that suggests a decline in hormone replacement therapy (HRT) use is linked to a fall in the number of new breast cancer cases.
Use of the drugs dropped rapidly after a series of studies claimed using HRT increases the risk of breast cancer.
Researchers subsequently suggested that the drop-off in HRT use was directly linked to a decline in breast cancer rates. But experts writing in the Journal of Family Planning and Reproductive Health Care said there is "no clear evidence" of a link.
However, the authors said that statistics and time trends "neither back up nor refute the claim".
The link between HRT and the disease has proved controversial, with several studies suggesting the finding has been blown out of proportion. But other research has reinforced the association, including two studies in 2006 and 2007.
One claimed that between 2002 and 2003 - following the publication of the reports which first raised concerns about the drug and subsequently the decline in use - the numbers of new breast cancer cases fell by 6.7%. The second study showed that the rates of single and combined HRT fell by 58% and 38% respectively between 2001 and 2003, while new breast cancer cases fell by around 11% in 2003.
But the authors of the new analysis point out that in both studies, the fall in breast cancer incidence started in 1999, three years before the sharp fall in HRT use began. They added: "Following the onset of genetic damage to breast tissue it has been estimated that, on average, at least a decade elapses before breast cancer becomes clinically detectable."
They go on to cite various methodological flaws in the studies that could have skewed the findings.
"Based on the observed trends in the incidence of breast cancer following the decline in HRT use, the ecological evidence is too limited either to support or refute the possibility that HRT causes cancer," they conclude.
In an accompanying editorial, Nick Panay chairman of the British Menopause Society, said that the controversy surrounding the link could "rage on for years" adding that scientists should not be distracted from the important task of helping women going through the menopause.