Selective Estrogen Receptor Modulators or SERMs are often referred to as "designer" estrogens.  Estrogen is able to enter the cell through what is termed a receptor site. Each cell is considered to have two receptor sites: an alpha receptor  that allows estrogen into the cell to produce a specific effect, and a beta receptor that acts to keep estrogen out of the cell.  If you think of each receptor site as a lock, then the ideal SERM would only open the alpha "lock " in the brain, bone, and cardiovascular system and produce a positive effect like estrogen.  Conversely, the ideal SERM would only open the beta "lock" in the breast and uterus, therefore providing protection like an antiestrogen. 

The ideal SERM would provide all of the benefits and none of the risks of estrogen. Unfortunately, there is no ideal SERM available yet, although there is a great amount of research in this area.

It may be a surprise to some that the phytoestrogens in soy protein may be considered SERMs. Soy acts like an estrogen and has a positive effect on the brain, bone, hot flashes, and the cardiovascular system.  Soy may also act like an antiestrogen to protect the breast and uterus.  Although not as potent as estrogen, soy protein has many benefits and no risks.

The most widely used SERM is tamoxifen, which is used to treat estrogen dependent breast cancer and to prevent breast cancer in those at high risk.  Acting like estrogen, tamoxifen has a negative effect on the uterus (increasing cancer and polyps), a negative effect in producing blood clots, and may have a slightly positive effect on the bone.  Tamoxifen is used mainly for its antiestrogen effect on the breast, but this antiestrogen effect also means that it can cause hot flashes and vaginal atrophy.

The drug that brought the terms "SERM" and "designer estrogen" into the main stream was raloxifene (Evista).  It was the first SERM developed for postmenopausal women.

Raloxifen acts like an estrogen to produce a positive effect on the bone and a mixed effect on the heart (lowering cholesterol but increasing clots).  It has been approved for the prevention of osteoporosis and has an effect on increasing bone mineral density and in reducing fractures. This effect is approximately half that of conjugated estrogens (Premarin 0.625 mg).  Although it does have a positive effect on the cardiovascular system, it is too soon to conclude that it should be used as a preventive measure. Current studies are underway.

Raloxifen acts like an antiestrogen in that it does not have a negative effect on the breast or the uterus.  Unfortunately because it acts like an antiestrogen, it does not help (and may even cause) hot flashes.  It is also not beneficial for vaginal atrophy.

It is probably too early to consider the SERMS like raloxifen as a replacement for HRT. However they do offer certain women options to conventional HRT. They may prove to be a valuable option for those with breast cancer or at risk for the disease.  SERMS may also be an option for women with osteoporosis risk who do not want to use estrogen and who cannot tolerate medications like Fosamax or Actonel.

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