Answer to a often asked question

Tamoxifen. What is it and how does it work? Here is the clearest explanation.

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How tamoxifen works
The way in which tamoxifen works is quite complicated and not yet fully understood, but its main function is as an anti-oestrogen drug.

Most breast cancers need supplies of the female hormone oestrogen to grow. Cancer cells have proteins called receptors on their surface that the sex hormones attach to. Cancers with oestrogen-receptors on the surface of their cells are called 'oestrogen-receptor-positive' (ER-positive) and tamoxifen is most effective against these cancers.

Under normal circumstances, when oestrogen comes into contact with the receptors, it fits into them and activates the cancer cells to divide so that the tumour grows.

Tamoxifen fits into the oestrogen receptor but does not activate the cells to divide. The tamoxifen stays in place and stops oestrogen from reaching the cancer cells so that they either grow more slowly or stop growing altogether.

Tamoxifen can greatly reduce the chance of oestrogen-receptor-positive cancers coming back after surgery. It can also be used to shrink large tumours before surgery so that they can be removed.