Forteo is a prescription drug used to treat osteoporosis in certain adults. The active ingredient is teriparatide, and it is used as a liquid solution injected under the skin. Forteo can cause mild to serious side effects. Common side effects include injection site pain, dizziness, and nausea. Serious side effects, such as bone cancer (osteosarcoma), are very rare. Most side effects are temporary and can be managed with simple methods like applying ice packs or taking over-the-counter pain relievers. If you experience severe side effects, contact your doctor or emergency services immediately. Forteo should not be used for more than two years.