Which medication is used primarily for osteoporosis and works by affecting bone density?

The medication primarily used for osteoporosis and that works by affecting bone density is Evista. It contains the active ingredient raloxifene, which is a selective estrogen receptor modulator (SERM). Evista mimics estrogen's beneficial effects on bone density without the associated risks that come with hormone replacement therapy. By binding to estrogen receptors in bone tissue, it contributes to the maintenance and increase of bone mass, which helps in reducing the risk of fractures in postmenopausal women.

In contrast, the other medications listed do not serve this purpose. Bisacodyl is a laxative used to treat constipation, Milk of Magnesia is an antacid and laxative, and Miralax is also a laxative. None of these options are indicated for osteoporosis or have any direct effect on bone density. Hence, Evista stands out as the correct answer in the context of medication for osteoporosis.