Many women with breast cancer are given the drug, tamoxifen, to reduce the risk of the disease recurring. However, now a new study has found that the drug also has a major preventive effect in women at risk of the disease, which remains constant for at least two decades.

According to the findings, the drug can reduce breast cancer rates by almost 30% over a 20-year period in women who are at high risk of developing the disease.

The study investigated the long-term benefits and risks of taking the drug to prevent breast cancer in women who were considered to be at high risk, e.g. women with a family history of the disease.

Over 7,000 women aged between 35 and 70 took part. The women were randomly chosen to receive 20mg of tamoxifen every day for five years, or a placebo. After the five years, the womens' health was assessed for an average of 16 years.

The study found that 350 of the women in the placebo group went on to develop breast cancer, compared to 251 in the tamoxifen group - a reduction of 29%.

Oestrogen receptor positive invasive breast cancer - which makes up around two-thirds of all cases of the disease - was reduced by 35%.

Furthermore, after 20 years of follow up, the estimated risk of developing cancer in the placebo group was 12%. However in the tamoxifen group, it was just 8%.

According to the study's lead author, Prof Jack Cuzick, of Queen Mary University in London, tamoxifen is already a ‘well-established and effective treatment for certain breast cancers, but we now have evidence of its very long-term preventive benefits'.

"The preventive effect of tamoxifen is highly significant with a reduction in breast cancer rates of around a third, and this impact has remained strong and unabated for 20 years. We hope these results will stimulate more women, particularly younger women, to consider treatment options for breast cancer prevention if they have a family history of the disease or other major risk factors," he commented.

He said that while a healthy diet and exercise can reduce the risk of breast cancer, for high risk women, ‘a more powerful intervention is needed'.

"This is where tamoxifen has a role to play."

However, the researchers warned that tamoxifen, like any other drug, will not work on everybody and carries a risk of side-effects, so it is essential that women discuss all of their options with their doctor.

Details of these findings are published in the journal, The Lancet Oncology.