A family of common hormone treatments can significantly reduce the risk of breast cancer in women at risk from the disease, research has shown.
One of the drugs, tamoxifen, is widely used to treat existing breast cancer after surgery. The other three, raloxifene, arzoxifene, and lasofoxifene, are all primarily treatments for the brittle bone disease osteoporosis.
A study of medical records for 83,000 women taking the pills showed that the drugs reduced breast cancer incidence by 38% in those at risk.
Researchers monitored the effect of taking the drugs for five years and then stopping treatment for a further five.
After five years of treatment the risk of breast cancer fell by 42%. But a reduction of 25% was also seen in women five years after they stopped the pills.
No effect was seen on numbers of deaths from breast cancer, or on cases of breast cancer not fuelled by the female hormone oestrogen.
The drugs, known as selective oestrogen receptor modulators (Serms) target oestrogen-sensitive molecules on cells in the breast and other parts of the body.
They can either block the hormone receptors or stimulate them. Some, such as raloxifene, can have a stimulating effect on bones while blocking oestrogen receptors in breast tissue.
Lead scientist Professor Jack Cuzick, from Queen Mary, University of London, who led the research reported in The Lancet medical journal, said: "These are very encouraging results and pave the way for more widespread use of these drugs in high risk women in a manner similar to the way statins and blood pressure lowering drugs are used to reduce the risk of heart disease and stroke."
Tamoxifen is currently not approved as a preventative treatment for breast cancer in the UK. In January the National Institute for health and Clinical Excellence (Nice), which vets the cost effectiveness of new treatments, provisionally recommended that tamoxifen should be given to women at high risk of breast cancer. Nice is due to issue its final guidance later this year.