Estrogen is a hormone which among other actions, regulates the turnover (formation and destruction) of bone. Decreases in estrogen levels that are seen after menopause or after removal of the ovaries, lead to a loss of bone density and weakened bones, a condition called osteoporosis. Raloxifene decreases bone turnover and increases bone density although not to the same extent as estrogen itself. This makes bones stronger and prevents fractures in women with osteoporosis. Raloxifene is called a "selective estrogen receptor modulator" since it has effects like estrogen on some tissues but inhibits the effects of estrogen on other tissues. Raloxifene decreases low density lipoprotein (LDL or "bad") cholesterol in the blood; however, unlike estrogen, raloxifene does not increase high density lipoprotein (HDL or "good") cholesterol.
Raloxifene belongs to a class of drug called the "selective (o)estrogen receptor modulators" or "SERMs". It is able to attach itself to specific tissues in the body which are stimulated by oestrogen (female-type hormone) including bone. One of oestrogens most important roles is to stimulate the production of new bone as part of the process by which the quality of the bones is maintained. Raloxifene, like oestrogen, can also stimulate target sites in bone tissue. When oestrogen levels fall (as they begin to after the change of life) the bones become thin or brittle with the result that fractures are likely to occur. This is one of the common ways by which osteoporosis, which is associated with reduced bone density, occurs. Raloxifene is therefore used to maintain or increase bone density and thereby reduce the occurrence of fractures in post-menopausal women (or younger women without ovaries) who are at risk of osteoporosis or in whom osteoporosis is established. A further interesting effect of Raloxifene is that it also binds to oestrogen target sites on the breast but in this case it blocks stimulation of breast tissue by oestrogen (a means by which many breast cancers develop). Raloxifene might therefore be considered as an alternative to HRT in osteoporosis if there are concerns about breast cancer because there is a strong family history.
Healthy bone tissue is continuously remodeled (broken down and then reformed); the minerals and other components of bone are reabsorbed by certain cells and then replaced by new bone formation. Raloxifene suppresses the activity of the cells that resorb bone; consequently, the breakdown of bone tissue occurs more slowly than the laying down of new bone. This action preserves bone density and strength.