Researchers say preventive effect of tamoxifen is highly significant with reduction in breast cancer rates of around a third

The preventative effect of the breast cancer drug tamoxifen is maintained for 20 years, reducing rates of the disease by about 30%, according to a study.

The authors of the research, led by Queen Mary University of London and funded by Cancer Research UK said the results “substantially improve the benefit-to harm ratio” for use of the hormonal drugs, which many complain have a number of side-effects, by women at high risk of breast cancer.

Professor Jack Cuzick, lead author of the paper, published in the Lancet Oncology on Thursday, said: “Tamoxifen is 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.”

During the study, 7,151 women aged from 35 to 70-years-old, at high risk of the disease – primarily with a family history of breast cancer – received, at random, either a 20mg daily dose of tamoxifen or a matching placebo for five years, the period for which the National Institute for Health and Care Excellence says the drug should be taken.

After completing treatment, their health was monitored with an average follow-up time of 16 years.

The results, presented at the San Antonio Breast Cancer Symposium, revealed that 350 women developed breast cancer in the placebo group compared to 251 of those taking tamoxifen, a reduction in occurrence of 29%. Oestrogen receptor (ER) positive invasive cancer, which account for two thirds of all breast cancers, were reduced by 35%.

The estimated risk of developing breast cancer was 8% in the tamoxifen group, compared to 12% in the placebo group, equating to 22 women being treated with the drug for every one breast cancer case prevented after 20 years.

Women who took hormone replacement therapy during the five years of tamoxifen treatment had a significantly lower benefit compared with those who did not. In addition, endometrial cancer, a known side effect of tamoxifen, was 3.8 times more common in the tamoxifen group during the five years of treatment (15 tamoxifen versus four placebo), but there was no increased risk in the follow-up period.

The reduced incidence of breast cancer with tamoxifen did not translate into a reduction in breast cancer specific deaths (31 tamoxifen versus 26 placebo) the researchers found. In addition, five women receiving tamoxifen died from endometrial cancer compared to none in the placebo group.

Cuzick, head of the Centre for Cancer Prevention at QMUL, said the number of breast cancer deaths was still small compared to the number of cases. “We will need to continue monitoring these women for a further decade to get a clearer picture of the impact of tamoxifen on death rates,” he said. “Some of the side effects of tamoxifen are also cause for concern and need continued monitoring – specifically the increased occurrence of endometrial cancer.”

The most common side effects of tamoxifen are menopausal-type symptoms, such as hot flushes and sweating. Other people feel sick or suffer indigestion or put on weight. Less common are depression, headaches, blood clots and vision problems.

Dr Julie Sharp, head of health information at Cancer Research UK, said the study had confirmed the “long-lasting effect” of tamoxifen but added: “All these drugs have side effects so it’s important that women at high risk of breast cancer talk through their choices with their doctor to work out the best option for them.”