Reclast is zolodronic acid, a bisphosphonate. Prolia is a monoclonal antibody. (Fosamax is a bisphosphonate.) If I read it correctly, the bisphosphonates were among the first drugs approved to treat osteoporosis but seem to falling out of favor. In part because of the rare side effect of osteonecrosis of the jaw, aka BRONJ aka bisphosphonate-related-osteonecrosis-of-the-jaw. (A few cases of which Prolia has since been associated with too now.)