Long-term use of one of the most common treatments for breast cancer dramatically increases the risk of developing a deadly secondary tumour, a study has shown.
Tamoxifen is the “gold standard” hormonal therapy given to thousands of British women to improve their chances of surviving breast cancer.
The drug prevents tumours being fuelled by the sex hormone oestrogen.
In women with hormone-sensitive cancers — who make up the majority of breast cancer patients — it can stop tumours returning after surgery.
But the new US research shows that using tamoxifen also raises the risk of developing more aggressive and difficult-to-treat new tumours that are not dependent on oestrogen.
The study found 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.
“This is of concern given the poorer prognosis of ER negative (oestrogen receptor negative) tumours which are also more difficult to treat,” study leader Dr Christopher Li, from the Fred Hutchinson Cancer Research Centre in Seattle, said.
Dr Li's team assessed the history of tamoxifen use among more than 1,000 women from the Seattle region who were diagnosed with hormone-sensitive breast cancer between the ages of 40 and 79.
The patients included 367 women diagnosed with both primary hormone-sensitive cancer and a second tumour and 728 who only had a first tumour.
Nearly all the women who underwent additional, or “adjuvant”, therapy after surgery were given tamoxifen.
Comparing patients who received tamoxifen and those who did not showed that the drug reduced the chances of oestrogen-positive breast cancer returning by 60%.
However, it also appeared to increase the risk of an oestrogen-negative second tumour developing by 440%.
The association was not seen for women who took tamoxifen for less than five years.
The scientists, whose findings are reported in the journal Cancer Research, said it was important to weigh up the risks and benefits of tamoxifen. They said the research did not suggest that women being treated with the drug should stop taking it.
“It is clear that oestrogen-blocking drugs like tamoxifen have important clinical benefits and have led to major improvements in breast cancer survival rates,” Dr Li said.
“However, these therapies have risks and an increased risk of ER negative second cancer may be one of them.
“Still, the benefits of this therapy are well established and doctors should continue to recommend hormonal therapy for breast cancer patients who can benefit from it.”