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 per cent 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 per cent. But a reduction of 25 per cent 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.
All four drugs significantly increased the risk of blood clots, the study showed. Only tamoxifen led to an increase in the rate of womb cancers.
Hazel Nunn, head of health information at Cancer Research UK, which funded the study, said: "These results provide some of the clearest evidence to date of the ability of these drugs to prevent breast cancer. The study also offers clarity on the frequency of side effects that can be expected from these drugs. Breast cancer is the most common cancer in the UK, and research like this has the potential to reduce the number of women diagnosed with the disease in the future.
"We look forward to the final guidance from Nice and hope this marks the time when women have more options to reduce their risk of breast cancer. These drugs have a range of side-effects though, so they will not be suitable for all women. We urge them to discuss their options with their doctors."
Dr Caitlin Palframan, from the charity Breakthrough Breast Cancer, said: "Draft guidelines from Nice recently took the historic step of recommending drug treatment to reduce the risk of breast cancer in women with a significant family history of the disease. Importantly, this may provide women with an alternative to risk-reducing surgery and we're pleased to see that the findings of this study further support this recommendation.
"In addition, this study looks at more drugs that could potentially be used to help prevent breast cancer and we look forward to seeing further research into their long-term effects.
"We expect Nice's final guidelines to be announced in June. Breakthrough Breast Cancer has been working with Nice to represent the views of patients and will strive to ensure the final recommendations are implemented swiftly."