As such from a chemical description of tamoxifen it is most definitely not a steroid type drug. In fact tamoxifen is a non-steroidal triphenylethylene,this family of compounds have shown steroidal mimicking characteristics. Tamoxifen can adopt a structural conformation which resembles the steroid nucleus. Some steriod compounds related to treatment of breast cancer are shown below. Hence in this conformation Tamoxifen can sit in a steroidal membrane and effectively block off the oestrogen receptor sites.
Tamoxifen competitively inhibits the binding of oestrogen to the oestrogen receptor sites.
As well as Tamoxifen directly blocking the oestrogen receptor sites there is also another important mechanism by which Tamoxifen can inhibit growth of cancerous cells. These cells possess the ability to hydroxylate the geminal phenyl ring and produce a Tamoxifen derivative 4-Hydroxytamoxifen. This is an example of a internal bodily reaction of which there are thousands. It is though that these lone pairs can effectively improve the binding of the derivative to the oestrogen receptor. This as in the case of Tamoxifen has the effect of blocking the site and vastly reducing oestrogen receipting properties.