Some breast cancers need the hormone oestrogen to grow. Tamoxifen is an 'anti-oestrogen' drug. This means that Tamoxifen slows or stops the growth of cancer cells that are already in the body and need oestrogen to grow. It does this by preventing the body's own oestrogen from entering the cancer cells. This deprives the cells of oestrogen and speeds up their natural death.
Tamoxifen is absorbed into the bloodstream and carried to all parts of the body.
In this way, it also reaches cancer cells that may have broken away from the main cancer and slows or stops them from growing. Tamoxifen is usually started about 1-4 weeks after surgery. If you have chemotherapy after surgery, Tamoxifen is started after chemotherapy is complete. Tamoxifen is taken as a tablet every day. Most women take it for 2 to 5 years.