Drug 'halves' breast cancer risk
12 December 2013 Last updated at 23:27 GMT
A "landmark study" shows a drug can more than halve the development of breast cancer in high-risk women.
A trial on 4,000 women, published in the Lancet, showed anastrozole was more effective, cheaper and had fewer side effects than current medications.
It stops the production of the hormone oestrogen, which fuels the growth of the majority of breast cancers.
Branwen Jeffreys reports.