Research has found one of the most common treatments for breast cancer increases the risk of developing a secondary tumour if used for a long time.
The new US research found that although tamoxifen is a "gold standard" hormonal therapy used to treat thousands of breast cancer patients, it raises the risk of developing more aggressive tumours, which are harder to treat.
Tamoxifen is commonly used in Britain to improve the chances of survival among patients diagnosed with breast cancer by blocking oestrogen to fuel the disease.
The drug has even been found to stop tumours returning after surgery when used to treat hormone-sensitive cancers.
But the latest study, by the Fred Hutchinson Cancer Research Centre in Seattle, shows that using tamoxifen also raises the risk of secondary tumours that are not dependent on oestrogen.
The findings reported in the journal Cancer Research show five or more years of treatment with the drug quadrupled the chances of an aggressive non-hormone sensitive tumour appearing opposite the initial site of the disease.
Study leader Dr Christopher Li said: "This is of concern, given the poorer prognosis of ER negative (oestrogen receptor negative) tumours, which are also more difficult to treat."
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