The study of 7,154 high-risk women, spanning over five years found in the 1990s that patients were 38% less likely to develop cancer and those who stopped taking it 20 years ago were still protected and was expected still to be working 30 years later or more.
British researchers said if GPs routinely prescribed Tamoxifen to the half a million women at high risk of breast cancer, as many as 3,000 cases a year would be prevented.
The pills are available on the NHS but fewer than 1,000 women are taking them to prevent breast cancer, partly because family doctors are unaware of the benefits.
Around one in eight women will develop breast cancer in Britain and there are 50,000 cases a year. Women are deemed to be high risk if their likelihood of getting the illness is greater than 17% most often due to a family history of the disease.
Lead researcher Jack Cuzick, a professor at Queen Mary, University of London, said that the drugs should be as widely prescribed as statins are to protect against heart disease.
“There’s a major cultural shift which the profession needs to embrace,” he said.
“There’s a fatalism about cancer, there’s a feeling there’s nothing you can do about it very much. We have very clear evidence we can do something.
He said many women were unaware they were at high risk and to address this, he said, they should take risk assessments – a short questionnaire – at the time of their first mammogram, aged 47 to 50.
Surprisingly, while it showed that Tamoxifen prevented the cancer occurring, the pill did not appear to reduce the total risk of dying from the illness.
Professor Cuzick believes that it is still too early for this to be apparent. The study is still running, and he suspects that in a few years it will be clear that Tamoxifen also prevents deaths from cancer.
Last year the NHS rationing body National Institute for Clinical Excellence recommended that GPs prescribe Tamoxifen to women at high risk. But bizarrely it has not been licensed as safe by watchdog, the Medicines and Healthcare Products Regulatory Agency which, as reported in the Daily Mail, is why many doctors are unwilling.
The drug works by blocking the hormone oestrogen from reaching potentially cancerous cells, which can cause them to divide. But it has unpleasant side effects, including many menopause-like symptoms such as hot flushes, sweats, nausea, muscle ache and weight gain.
There is also a small risk of endometrial cancer, a rare form of cancer in the womb lining.