The anti-breast cancer drug tamoxifen may cause a rare form of uterine cancer, authorities have warned.

The Medicines Controls Agency in the UK and the Food and Drugs Administration in the US have warned the drug may increase the risk of developing uterine sarcoma, a rare but aggressive cancer.

The warning is directed at women who have not had breast cancer but take the drug because they are at high risk of developing the disease.

Information for prescribers and patients has already been updated in the UK

MCA spokesman

In the UK, the drug can only be prescribed to women who have been diagnosed with breast cancer although there have been calls for it to be made available to at risk groups.

The MCA said it has updated patients' and doctors' information leaflets advising them of the risk. The FDA has taken similar steps.

Benefits

However, both authorities have said that benefits of tamoxifen far outweigh the risks for patients who have already been diagnosed with breast cancer.

A spokesman for the MCA said: "In the UK, tamoxifen is licensed for the treatment of women with existing breast cancer.

"Information for prescribers and patients has already been updated in the UK to include a warning about the increased risk of uterine sarcoma with use of tamoxifen.

"As with all marketed medicines the safety of tamoxifen is continuously monitored by the Medicines Control Agency/Committee on Safety of Medicines."

The FDA urged doctors to discuss the risks with patients.

In a statement, it said: "Healthcare providers should discuss the potential benefits versus the potential risks of these serious events with women considering [tamoxifen] to reduce their risk of developing breast cancer."

Effective treatment

International studies have shown that tamoxifen is a very effective treatment for women with breast cancer.

First developed in the UK, the drug is relatively cheap and helps prevent the cancer coming back after the initial surgery to remove a breast tumour. It is usually prescribed for five years.

Scientists have found that it is also effective as a preventative treatment, reducing incidence of the disease by as much as one third.

A spokesman for AstraZeneca, which manufactures tamoxifen for the UK, said: "We are well aware of the risks, as these are well documented.

"That is why we are looking to produce new chemical compounds to replace tamoxifen - clinical trials are underway."