The drug, first developed in the UK, is cheap and helps prevent the cancer coming back after the initial surgery to remove a tumour.
Tamoxifen blocks the female sex hormone oestrogen.
The hormone influences the growth of cells related to female reproduction, such as those in the breast or the uterus. If there is too much oestrogen in the system, cell growth can accelerate to the point where tumours start to develop. When the drug locks onto the receptors it blocks the way for oestrogen - which would otherwise activate the cancer cell to divide and make the tumour grow. Tamoxifen, known in the trade as Nolvadex, is usually prescribed by specialists in breast cancer and is taken in pill form. A patient will stay on the drug for about five years.