Q. My family history of breast cancer (both my mother and sister have had it) puts me at high risk of the disease. And now that I'm pushing 60, I'm starting to be concerned about osteoporosis, too. My doctor has recommended I take Evista, which she said would help on both fronts. What is it?

A. Evista (raloxifene) has been a very common anti-osteoporosis drug for years. It's known as a SERM (short for selective estrogen-receptor modulator, should you ever hear the term thrown around). SERMS, which also include tamoxifen and Fareston (toremifene), work by blocking estrogen from attaching itself to the special receptors on cancer cells. Think of the four-way electrical outlet in your kitchen; if the toaster, microwave, coffeemaker, and a radio are all plugged in, well, there's just no room for the electric frypan. A SERM "fills" a cell's estrogen receptors, leaving no room for estrogen. Breast cancer cells deprived of their estrogen go dormant, and some even die immediately.

Interestingly, the manufacturer of Evista, Eli Lilly, was fined several years ago for promoting it as an anti-breast cancer drug, before it had gained FDA approval for that use. As of September 2007, the FDA has given that approval, saying it can be recommended for postmenopausal women at high risk of breast cancer.

As far as osteoporosis, Evista will take over where estrogen has left off. As you've aged, your estrogen level has declined (but you knew that, right?). Estrogen is key in controlling the balanced growth of bones; your bones are constantly being "remodeled," with older bone being destroyed, and new bone taking its place. Without estrogen to help keep these two functions in balance, sometimes old bone is destroyed faster than new bone can be made. The result? Weakened, brittle, thin bones: osteoporosis.

As a SERM, Evista will bind to the estrogen receptors on your cells, and then "act" like estrogen with your bone cells, keeping their remodeling in balance. At the same time, it's preventing breast cells from binding with estrogen, which also prevents them from becoming cancerous. Pretty neat, huh? Sometimes having a "split personality" is an asset!

Q. I have friends who've had breast cancer and compare Evista to tamoxifen. Is that a choice I should be looking into?

A. No, not if you haven't had breast cancer. Evista hasn't been found to be effective in preventing recurrence in women who've already had breast cancer; it's for women at high risk for other reasons, be it genetic, dense breasts, lifestyle, or some other known risk factor. A clinical trial that concluded in 2006, the STAR trial-the largest breast cancer prevention trial ever conducted-showed that tamoxifen and raloxifene (Evista) were equally effective at preventing breast cancer in high-risk women, and that Evista had fewer dangerous side effects-unless the woman was also dealing with cardiovascular issues. Unlike tamoxifen, Evista doesn't carry an increased risk for endometrial or uterine cancer.