An OESTROGENS receptor antagonist – that is, the drug blocks the action of oestrogen. It is effective in women with early steroid-hormone sensitive breast cancer (about 75% of all such cancers) as extra treatment after surgery. It eradicates micrometastases, that is tiny collections of breast cancer cells lying outside the treated breast and reduces the risk of cancer in the remaining breast. In advanced breast cancer it is suitable for certain groups of postmenopausal women but others may be advised to take aromatase inhibitors such as anastrazole and letrozole (see BREASTS, DISEASES OF).

In patients with tumours that are oestrogen-sensitive, the positive response to tamoxifen is 60 per cent; those tumours that are not oestrogen-sensitive are much less likely to respond to the drug. Tamoxifen (and raloxifene) increases both survival rates and the period between the diagnosis of the tumour and appearance of metastatic growth (see METASTASIS) in tumours sensitive to it. It can also prevent the onset of breast cancer in women known to be liable to the disease because of their genetic make-up. The drug has fewer adverse effects than most others used for treating breast cancer, although it may cause liver problems in women who are obese. Tamoxifen is also used to treat INFERTILITY in patients with POLYCYSTIC OVARY SYNDROME.