Mammograms screen for cancer

Trials showed the drug can prevent the occurrence of cancer but the side effects for most women are severe, a University of North Carolina team found

Its preventive quality was first raised when women treated for cancer in one breast found the tumour did not spread.

The team's calculations are published in the Archives of Internal Medicine.

Powerful drug

Tamoxifen has proved its worth as a means of stopping the spread or recurrence of breast cancer in women who have already been treated for it.

But, it was noticed back in the early 1980s that some women who were receiving the drug for cancer in one breast did not develop any cancer in the other breast.

This prompted the suggestion that tamoxifen might have another preventative role for those women who are at risk of getting breast cancer but have yet to develop any signs of the disease.

Trials have shown tamoxifen does prevent breast cancer from occurring in the first place.

It works by blocking the female hormone oestrogen which is known to encourage breast tumour growth in some women.

The side effect profile of tamoxifen in women who have not had breast cancer means that this is not the answer Dr Michelle Barclay from Breakthrough Breast Cancer

But being a potent drug with known side effects, it is not known how many women would safely benefit from any protective effect.

Dr Russell Harris and colleagues asked 605 women aged 40 to 69 attending a GP surgery to fill out questionnaires about their health and family histories of breast cancer.

Then, using a proven formula for assessing breast cancer rates they estimated how many of these women would be likely to go on to develop breast cancer over the next five years.

Most of the women were judged to be at very low risk of developing breast cancer.

In others, tamoxifen therapy would not have been suitable because of existing conditions like high blood pressure or diabetes.

Trials

Therefore, the percentage of actual cancers prevented by tamoxifen would have been very small - between 6% and 8.3%, according to the researchers.

Trials are ongoing and will provide vital information helping doctors choose the best strategy to prevent breast cancer in women at greatest risk Dr Julie Sharp from Cancer Research UK

Dr Harris said: "Tamoxifen's possibly harmful side effects, including blood clots and strokes, would rule out some 90% of women who might benefit from taking it each day."

Dr Michelle Barclay from Breakthrough Breast Cancer says: "Many women want to prevent breast cancer, especially those who are at increased risk due to their family history of the disease.

"However, the side effect profile of tamoxifen in women who have not had breast cancer means that this is not the answer."

She said it was clear that more research into preventing breast cancer with drug treatments was needed.

She said researchers were currently looking at whether a drug called anastrozole could prevent breast cancer.

Like tamoxifen, this drug blocks the action of oestrogen, but it tends to have fewer side effects.

This work is funded by Cancer Research UK, which is also investigating the use of tamoxifen to prevent breast cancer in high-risk women.