A previous clinical trial found that tamoxifen, taken for five years, reduced the risk of developing invasive breast cancer by about 50% in post-menopausal women who were at increased risk of getting the disease.
Another trial found that five years of raloxifene reduces breast cancer risk in such women by about 38%.
Under the new plans, high risk post-menopausal women could be offered the drugs for a period of five years unless they have a history of thromboembolic disease or endometrial cancer.
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Between them, the two key EU regulators currently employ around 1,000 people in Canary Wharf.