A new drug could help slow half of breast cancer cases, a study suggests. The hormone progesterone, which is both cheap and safe, was found to slow the growth of some tumours.
British and Australian authors of the study, published in Nature, described the findings as "very significant" and are now planning to run clinical trials.
Scientists are already aware that hormones play an important role in breast cancer, making a cancerous cell divide by hooking up with "hormone receptors".
Tamoxifen, one of the most effective cancer drugs, prevents the oestrogen receptor from working.
Scientists found that cancer cells growing in the laboratory grew to half the size when treated with progesterone and tamoxifen than when given tamoxifen alone.
Researcher, Prof Carlos Caldas from the University of Cambridge, told the BBC News website: "It appears you control the tumours better, but to prove it is better in women with breast cancer we need to do the trial.
"It could be very significant. In early breast cancer you could increase the number of people being cured and in advanced breast cancer, where we're not curing, we could control the disease for longer."
Cancer Research UK said the study was "highly significant" and could help thousands of women. Dr Emma Smith said: "This is a highly significant finding. It could be an easy, cheap and simple way to improve the survival of thousands of women, but it needs clinical trials."