 Methotrexate is one of the most effective chemotherapy drugs in the treatment of osteosarcoma. However, it has severe side effects. The physicians tries to save or rescue normal cells from the side effects of methotrexate by giving folinic acid. Folinic acid administration starts 24 hours after methotrexate and given regularly until methotrexate levels are low and not dangerous to normal cells anymore. Despite this rescue, side effects are still a problem and many patients are not well enough to receive subsequent chemotherapy on time. 