Teriparatide, sold under the brand name Forteo, is a form of parathyroid hormone (PTH) consisting of the first (N-terminus) 34 amino acids, which is the bioactive portion of the hormone. It is an effective anabolic (promoting bone formation) agent used in the treatment of some forms of osteoporosis. It is also occasionally used off-label to speed fracture healing. Teriparatide is identical to a portion of human parathyroid hormone and intermittent use activates osteoblasts more than osteoclasts, which leads to an overall increase in bone. Medical uses It is effective in growing bone (e.g., 8% increase in bone density in the spine after one year) and reducing the risk of fragility fractures. When studied, teriparatide only showed bone mineral density (BMD) improvement during the first 18 months of use. Teriparatide should only be used for a period of 2 years maximum. After 2 years, another agent such a bisphosphonate or denosumab should be used in cases of osteoporosis although repeat administration has been used in hypophosphatasia, where bisphosphonates are contraindicated. Teriparatide cuts the risk of hip fracture by more than half but does not reduce the risk of arm or wrist fracture. Contraindications Teriparatide should not be prescribed for people who are at increased risks for osteosarcoma. This includes those with Paget's Disease of bone or unexplained elevations of serum alkaline phosphate, open epiphysis, or prior radiation therapy involving the skeleton. In the animal studies and in one human case report, it was found to potentially be associated with developing osteosarcoma in test subjects after over 2 years of use. In 2021, the United States Food and Drug Administration removed the time limit for treatment with teriparatide, if a patient remains at or has returned to having a high risk for fracture. Patients should not start teriparatide until any vitamin D deficiency is corrected. Adverse effects Adverse effects of teriparatide include headache, nausea, dizziness, and limb pain. Teriparatide has a theoretical risk of osteosarcoma, which was found in rat studies but not confirmed in humans. This may be because, unlike humans, rat bones grow for their entire life. The tumors found in the rat studies were located on the end of the bones which grew after the injections began. After nine years on the market, there were only two cases of osteosarcoma reported. This risk was considered by the FDA as "extremely rare" (1 in 100,000 people) and is only slightly