The British pharmaceuticals and chemicals giant Zeneca Group Plc, who market the drug under the name Nolvadex, say it's the first drug to get US Food and Drug Administration approval as a breast cancer preventative.

"Breast cancer is the second leading cause of cancer death in American women and accounts for 31% of all cancers among women.

Women at risk are scanned for the disease in the UK

"Today's action provides an important new option for some women at heightened risk of breast cancer," said Health and Human Services Secretary Donna Shalala in a statement on Thursday.

In the US, women have a one in eight chance of developing breast cancer.

Women are considered high risk for reasons including a family history of the disease and a history of breast lumps.

The FDA said Zeneca would help doctors assess which women are at high risk and might benefit from the drug.

Tamoxifen has been shown in trials to reduce the incidence of breast cancer in healthy women by 49%.

'Risks associated with use'

But doctors say they still do not know if it prevents cancer or just delays its onset.

A National Cancer Institute trial of more than 13,000 women was interrupted early last April when the results became clear.

But there are warnings about the drug, which can cause an increase in the rate of rare uterine cancer.

FDA commissioner Dr Michael Friedman said: "Caution must be used in prescribing the drug because of its potentially serious side effects including endometrial cancer [cancer of the uterus], deep vein thrombosis [blood clots in major veins] and pulmonary embolism [blood clots in the lungs].

"As with all drugs, there are risks associated with use.