A key treatment for women with breast cancer is the use of drugs that modify the effect of the female hormone oestrogen. This treatment has side-effects of poor memory and concentration, which can discourage women from completing the course. Having adequate magnesium in the body is important for normal brain function, and is influenced by an effect of oestrogen on the kidney. The question is whether these side effects could be reversed by a magnesium supplement. This study is designed to test the feasibility of a larger clinical trial which would compare two groups of women on breast cancer hormone treatments, one randomly assigned to having magnesium tablets daily for 3 months and the other assigned to placebo (‘sugar-pill’) tablets for 3 months. Magnesium levels and brain function tests of the two groups would be compared. A finding that some women benefit from this simple remedy would have enormous global impact.