The drug is well-known as a treatment for existing breast cancer, but some doctors suggest that healthy women should take it to prevent the disease developing in the first place.

However, scientists at the University of Southern California have found that women taking the drug for five years or more increased the chances of developing endometrial cancer by 400%.

Those at highest risk were those women also taking oestrogen-only hormone replacement therapy, and obese women.

Balancing risks and benefits

The results mean that while the benefits to breast cancer patients of taking tamoxifen are clear-cut when compared with the risks, some doctors are less certain about giving the drug to healthy women.

Dr Anthony Swerdlow, who is involved in more research looking at links between tamoxifen and other cancers, said: "I am sure the debate isn't over.

Breast cancer is more common than endometrial cancer

"If a four-fold increased risk has been found, that will weigh appreciably on people's minds when they are considering whether to take tamoxifen to try and prevent breast cancer."

He said one of the founding principles of medicine was to "do no harm" - and that giving the drug to healthy patients might well do that.

The normal risk of developing endometrial cancer, which affects the lining of the womb, is far lower than getting breast cancer, and it is also more treatable with surgery and chemotherapy.

Dr Leslie Bernstein, who led the study, published in the Journal of the National Cancer Institute, said: "We have confirmed the findings of other studies showing that tamoxifen increases the risk of uterine cancer, and most importantly, we have found that women who have used oestrogens and who are overweight have the greatest risk."

"Our results suggest that physicians should be particularly vigilant in monitoring tamoxifen-treated patients with these additional risk factors."

A recent study indicated that tamoxifen appeared to be effective against cancer even when given in much smaller doses.