A new US study following the medical outcomes of 9,000 women over 50 across eight years found that those taking tamoxifen to control cancer in one breast had a greatly increased risk of developing a tumour in the other. They found that, while tamoxifen apparently protected against oestrogen-receptor-positive tumours, it appeared to increase the risk of an oestrogen-receptor-negative tumour in the healthy breast fivefold.

Oestrogen-receptor-negative tumours are more difficult to treat, have a high death rate and an 8%-35% lower 5-year survival rate.