Researchers report they have conclusive proof that tamoxifen, the most widely prescribed drug for treating breast cancer, can also prevent breast cancer among healthy women who are at a high risk of the disease.

The researchers reported in the journal The Lancet that tamoxifen reduced the incidence of breast cancer by 38 per cent in healthy women.

"In our analysis we combined all the available evidence from studies using tamoxifen for breast cancer prevention collectively involving over 40,000 women -- and it is clear to us now that the drug can reduce the chance of high-risk women developing the disease," said Professor Jack Cuzick of Cancer Research UK.

The researchers reported that the drug, which counters the action of estrogen which stimulates breast cancer growth, also was highly effective for breast cancer survivors whose tumors are sensitive to estrogen, reducint the incidence of new cancer in the opposite breast by 46 percent.

Tamoxifen is ineffective against tumors that are not sensitive to the hormone.

The researchers reported that more research is needed to minimize the side effects of tamoxifen – such as blood-clotting disorders – before it can be widely used as a preventative drug.

They also reported very promising signs in early trials that the drug raloxifene can reduce the risk of breast cancer by 64 per cent while avoiding the side-effects of tamoxifen

Other sources: The Lancet