The purpose of this study is to compare the effects of two teriparatide products, PF708 and Forteo, in patients with osteoporosis.

This is a randomized, multi-center study conducted in the United States. Men and women with osteoporosis will be enrolled in a parallel-group, open-label study design to compare the effects of PF708 and Forteo after 24 weeks of treatment. Half of the subjects will be randomized to receive PF708, and the other half will be randomized to receive Forteo.

Inclusion Criteria:

• If female, ≥5 years postmenopausal at the time of screening, with a DXA-derived BMD value at least 1 standard deviation (SD) below the average of young, healthy women
• If male, has a DXA-derived BMD value at least 2 SD below the average of young, healthy men
• Able to use the pen injection device correctly
• Able to understand and sign the written Informed Consent Form (ICF)

Exclusion Criteria:

• Treatment with oral bisphosphonates (once daily or once weekly) within 6 months of screening
• Any current or prior human PTH-derived products (e.g., Forteo, Teribone, Natpara), including for investigational purposes
• Immobility due to severe or chronically disabling conditions (e.g., stroke, Parkinson's disease, multiple sclerosis)
• History of metabolic bone diseases other than osteoporosis
• History of malignant disease, including solid tumors and hematologic malignancies (except basal cell and squamous cell carcinomas of the skin that have been completely excised and are considered cured)
• History of Paget's disease of bone
• History of prior external beam or implant radiation therapy involving the skeleton
• Active urolithiasis or primary hyperparathyroidism