A DRUG costing less than £30 a year can slash the risk of developing breast cancer, a worldwide study has found.

Tamoxifen is normally used to treat breast cancer - but researchers have now found it can also provide protection against developing the disease for several years.

Doctors said the drug could become an option for healthy woman at high risk of breast cancer.

Research published in 2002 showed tamoxifen could cut the risk of breast cancer by one third.

Now follow-up analysis from the same group has found the protection continues for at least five years after women stop taking the pills.

The research, published in the Journal of the National Cancer Institute, also found side-effects from the drug, such as blood clots, stopped, while the protection continued.

The findings come from the International Breast Cancer Intervention Study, involving 7000 women with risk factors, such as a family history of breast cancer.

The study, funded by Cancer Research UK, involved the women taking either 20mg of tamoxifen every day for five years, or a dummy pill.

A total of 142 of the women who had taken the tamoxifen were later diagnosed with breast cancer, but among those who took the placebo the number was 195 - a difference of 27%.

Among certain types of cancer the difference was 34%.

Professor Tony Howell, of South Manchester University Hospitals Trust and co-chairman of the study group, said: "Previous studies have already shown that tamoxifen lowers the risk of developing breast cancer.

"But this is the first time clear evidence is available on the benefits and side-effects of tamoxifen after treatment with the drug has stopped.

"These findings, together with the effectiveness results, suggest that over a longer follow-up time the risk of side-effects decreases, while the benefit of prevention continues."

Kate Law, director of clinical trials at Cancer Research UK, said: "These results are promising for women at increased risk of breast cancer.

"They are important in furthering our knowledge of the role of tamoxifen in the prevention of the disease and also raising the possibility of making it available on a wider scale."

Tamoxifen has been used to treat breast cancer for more than 20 years and treatment costs just 8p a day.

Meanwhile, post-menopausal women are being recruited for a new international study to examine whether the drug anastrozole has fewer side-effects than tamoxifen.