Tamoxifen
 Tamoxifen is given to most women with breast cancer |
Long-term use of a common breast cancer drug may hike the risk of developing a deadly second tumour, a study suggests.
Tamoxifen, given to thousands of British women, prevents tumours being fuelled by the sex hormone oestrogen, and stops them returning after surgery.
But a US study links use of the drug to a four-fold raised risk of developing a more aggressive, difficult-to-treat tumour, not dependent on oestrogen.
However, women are strongly advised not to stop taking tamoxifen.
 |  Women should be reassured that the benefits of taking hormone-blocking drugs, such as tamoxifen, after their first diagnosis of breast cancer far outweigh any potential risks  Dr Alison Ross Cancer Research UK |
Experts stress any risks of taking the drug are far outweighed by the benefits.
They said the odds of developing a second, non-hormone sensitive tumour remained very low.
Each year around 45,500 women in the UK are diagnosed with breast cancer and 12,000 die from the disease.
Around two thirds of breast cancers are sensitive to the hormone oestrogen.
Tamoxifen become the "gold standard" treatment for these hormone-sensitive tumours, although in recent years newer drugs have started to be preferred.
The latest study, by the Fred Hutchinson Cancer Research Center in Seattle, looked at long-term use of the drug among more than 1,000 women.
The researchers, writing in the journal Cancer Research, found that tamoxifen reduced the chances of oestrogen-positive breast cancer returning by 60%.
But they also found that five or more years of treatment was associated with a 440% increase in the chance of an aggressive, non-hormone sensitive tumour appearing in the opposite breast.
These tumours can be particularly difficult to treat.
Many women in the UK cease tamoxifen treatment after five years to avoid side effects, but several thousand woman have been on the drug for a longer time.
Risks and benefits
Lead researcher Dr Christopher Li said: "It is clear that oestrogen-blocking drugs like tamoxifen have important clinical benefits and have led to major improvements in breast cancer survival rates.
"However, these therapies have risks, and an increased risk of ER negative (oestrogen receptor 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."
Professor Jack Cuzick, head of Cancer Research UK's Centre for Epidemiology, Mathematics and Statistics at Queen Mary, University of London, stressed that tamoxifen had a proven track record.
He said: "There is overwhelming evidence that tamoxifen, and newer more effective hormone blocking treatments, prevent far more recurrences, new breast cancers and cancer-related deaths than they might stimulate."
Professor Cuzick said some of the non-hormone sensitive tumours recorded in the study may have started out as hormone-sensitive, but had been kept at bay by tamoxifen treatment.
Dr Alison Ross, senior science information officer at Cancer Research UK, said: "Women should be reassured that, based on extensive scientific evidence, the benefits of taking hormone-blocking drugs, such as tamoxifen, after their first diagnosis of breast cancer far outweigh any potential risks.
"More research will be needed to confirm the possible link between its long-term use and the relatively rare occurrence of an aggressive form of the disease in the other breast."
