Medical charities have reassured women over the benefits of taking tamoxifen after research suggested the breast cancer drug could, in certain circumstances, increase the risk of secondary tumours.

Tamoxifen, which prevents tumours being fuelled by oestrogen, is one of the most common treatments given to improve patients' chances of surviving breast cancer. In women with hormone-sensitive cancers, who make up the majority of breast cancer patients, it can stop tumours returning after surgery.

The new US research, however, shows that using tamoxifen for more than five years may raise the risk of developing more aggressive tumours not dependent on oestrogen. The study found that such extended use quadrupled the chances of an aggressive, non-hormone sensitive tumour appearing opposite the initial site of the disease.

Dr Christopher Li, who led the research team at the Fred Hutchinson cancer research centre in Seattle, said: "This is of concern, given the poorer prognosis of ER negative [oestrogen receptor negative] tumours, which are also more difficult to treat."

The study 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.

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," said Li. "However, these therapies have risks, and an increased risk of ER negative second cancer may be one of them."

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.

Professor Jack Cuzick, head of Cancer Research UK's centre for epidemiology, 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.

"Work on understanding how tumours eventually escape from hormone treatment will be very valuable. However, the use of hormone blocking drugs to treat oestrogen receptor positive breast cancer will continue to be the main form of treatment."

Dr Alison Ross, the charity's senior science information officer, 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."

Dr Sarah Cant, the policy manager at Breakthrough Breast Cancer, said: "This research confirms the benefits of taking tamoxifen, which has played a major part in improving survival rates for breast cancer. Although it suggests women who take tamoxifen for five years or more may be at a slightly increased risk of a specific type of breast cancer in the other breast, the overall risk of this happening is still low.

"Women with breast cancer should be aware of the risks of taking any course of treatment. Breakthrough advises that anyone who is concerned should not stop their medication and should speak to their doctor."