FDPS and Osteopenia: GPP can be condensed again with an IPP molecule to form farnesyl pyrophosphate (FPP; 15 carbon), with the help of the FPP synthase (FPPs), the enzyme target of bisphosphonates (e.g., alendronate, risedronate), which is widely used as a drug in the treatment of osteopenia and osteoporosis.