Alendronate belongs to a class of drugs called bisphosphonates. Bisphosphonates are used to treat conditions that affect your bones including osteoporosis.
Bisphosphonates work on your bone-making cells. They slow down how quickly you lose bone. They can help to restore some lost bone and help to prevent further bone loss. People who take a bisphosphonate are less likely to break or fracture a bone.
(In answer to your question they slow down bone loss.)
Like all drugs, bisphosphonates have risks as well as benefits. For well over a decade, though, doctors have prescribed bisphosphonates to millions of people, very few of whom developed major side effects.
Still, reports of an association between bisphosphonate use and two rare but serious problems — osteonecrosis of the jaw and a specific type of fracture in the long bone of the leg (femur) — have raised concerns. Osteonecrosis of the jaw develops when the jaw bone fails to heal after a minor injury — for example, the type of injury sometimes caused by getting a tooth pulled.
A clear cause-and-effect relationship between bisphosphonate use and jaw osteonecrosis remains unproved. The best estimate of risk is 0.1 percent, meaning the condition occurs in 1 out of every 1,000 taking bisphosphonates over the course of a year. The risk of a bisphosphonate-related leg fracture seems to be much lower than that of jaw osteonecrosis.
Overall, bisphosphonates are more likely to help you by preventing fractures than to hurt you by causing jaw problems or unusual femur fractures. This is particularly true for women with postmenopausal osteoporosis.
Hope this helps.