Teriparatide (Forteo): The PTH Peptide That Builds Bone

Bone-Building Peptides

65% fewer vertebral fractures

In the landmark 2001 trial of 1,637 postmenopausal women, daily teriparatide cut new vertebral fracture risk by 65% compared to placebo.

Neer et al., NEJM, 2001

Neer et al., NEJM, 2001

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Most osteoporosis drugs slow bone loss. Teriparatide does the opposite: it tells the body to build new bone. Approved by the FDA in 2002 under the brand name Forteo, teriparatide is a synthetic version of the first 34 amino acids of human parathyroid hormone (PTH). In a trial of 1,637 women, it reduced new vertebral fractures by 65% and increased lumbar spine bone mineral density (BMD) by 9 percentage points over placebo.^[1] It was the first drug to treat osteoporosis by actually forming bone rather than just preventing its breakdown. For a broader look at how parathyroid hormone and bone interact, including why a hormone known for dissolving bone can also build it, the cluster pillar on bone-building peptides for osteoporosis covers the full landscape.

What Is Teriparatide?

Teriparatide is recombinant human PTH(1-34), meaning it consists of the first 34 amino acids of the 84-amino-acid parathyroid hormone. This N-terminal fragment is the biologically active portion that binds to the PTH type 1 receptor (PTH1R) on osteoblasts, osteocytes, and renal tubular cells.^[1]

Produced by recombinant DNA technology in E. coli, teriparatide is administered as a once-daily subcutaneous injection of 20 micrograms from a prefilled pen device. Each pen contains a 28-day supply. The original FDA-approved treatment duration was limited to 24 months based on preclinical safety concerns, though this restriction was relaxed after the 2020 label update.^[5]

The drug is approved for postmenopausal women with osteoporosis at high fracture risk, men with primary or hypogonadal osteoporosis, and men and women with glucocorticoid-induced osteoporosis. The biosimilar Bonsity (approved 2023) offers the same molecule at a lower cost.

The Intermittent PTH Paradox: How Teriparatide Builds Bone

Here is the counterintuitive part. Parathyroid hormone, in its natural role, dissolves bone. When PTH levels stay elevated continuously (as in hyperparathyroidism), osteoclasts dominate and bone density drops. But when PTH hits bone cells in brief, intermittent pulses, the response flips. Osteoblasts activate, proliferate, and survive longer. Bone formation outpaces resorption.

Teriparatide exploits this paradox through three mechanisms:

Direct osteoblast stimulation. Binding to PTH1R activates the cAMP/protein kinase A pathway and the protein kinase C pathway. These cascades increase osteoblast number by stimulating proliferation and reducing apoptosis.^[1]

Remodeling-based formation. At existing remodeling sites where old bone is being replaced, teriparatide shifts the balance toward formation. More bone gets deposited than removed during each remodeling cycle.

Modeling-based formation. Teriparatide also triggers new bone formation on previously quiescent bone surfaces, a process called modeling. This is unusual for a drug and contributes to the large gains in trabecular bone architecture. For a deeper look at this mechanism, see why parathyroid hormone can build the same bone it dissolves.

The net effect is particularly strong in trabecular bone (the spongy interior of vertebrae and the hip), which explains why spine BMD gains consistently exceed hip BMD gains in clinical trials.

The Landmark Fracture Prevention Trial

The trial that established teriparatide was published in the New England Journal of Medicine in 2001 by Neer and colleagues.^[1] It remains the foundational dataset for this drug.

Design: 1,637 postmenopausal women with prior vertebral fractures received daily subcutaneous injections of 20 mcg teriparatide, 40 mcg teriparatide, or placebo. Median observation was 21 months.

Fracture results at 20 mcg:

• New vertebral fractures: 5% vs 14% placebo (relative risk 0.35, 95% CI 0.22-0.55)
• New nonvertebral fragility fractures: 3% vs 6% placebo (relative risk 0.47, 95% CI 0.25-0.88)
• Moderate-to-severe vertebral fractures: 90% risk reduction

BMD gains at 20 mcg:

• Lumbar spine: +9 percentage points over placebo
• Femoral neck: +3 percentage points over placebo
• Total body bone mineral: +2 to 4 percentage points over placebo

The 40 mcg dose produced greater BMD gains (13 points at spine) but did not reduce fractures further and caused more side effects, particularly nausea and headache. The 20 mcg dose became the standard.

The trial was stopped early (median 19 months instead of the planned 36) because of a preclinical finding in rats, which triggered the osteosarcoma black box warning discussed below.

Teriparatide Outperforms Bisphosphonates in Head-to-Head Trials

Two pivotal trials compared teriparatide directly against the standard of care.

Glucocorticoid-Induced Osteoporosis (Saag 2007)

Saag and colleagues randomized 428 men and women on chronic glucocorticoids to teriparatide 20 mcg/day or alendronate 10 mg/day for 18 months.^[2]

Results were unambiguous. Lumbar spine BMD increased 7.2% with teriparatide versus 3.4% with alendronate (P<0.001), with the difference significant by 6 months. New vertebral fractures occurred in 0.6% of the teriparatide group versus 6.1% of the alendronate group (P=0.004). For patients whose bones are being actively damaged by glucocorticoids, teriparatide offered meaningfully better protection than the bisphosphonate standard.

The VERO Trial (Kendler 2018)

The VERO trial was the first head-to-head fracture endpoint trial between an anabolic agent and an antiresorptive. Kendler and colleagues randomized 1,360 postmenopausal women with severe osteoporosis (at least two moderate or one severe vertebral fracture) to teriparatide 20 mcg/day or risedronate 35 mg/week for 24 months.^[4]

New vertebral fractures occurred in 5.4% of teriparatide patients versus 12.0% of risedronate patients (risk ratio 0.44, 95% CI 0.29-0.68, P<0.0001). Clinical fractures (a composite of clinical vertebral and nonvertebral fractures) were also lower with teriparatide (4.8% vs 9.8%, P=0.0009).

This was the first randomized trial to demonstrate superior fracture reduction for an anabolic agent over an antiresorptive. It changed treatment guidelines: several societies now recommend starting with an anabolic agent in patients at very high fracture risk, rather than defaulting to bisphosphonates first. For a detailed comparison of teriparatide and abaloparatide, see the dedicated analysis.

Does Teriparatide Accelerate Fracture Healing?

Beyond preventing new fractures, researchers have investigated whether teriparatide speeds healing of existing ones. A 2016 meta-analysis by Lou and colleagues pooled data from 5 randomized controlled trials with 251 osteoporotic patients.^[7]

The results were modest but detectable: radiological fracture healing time was 4.54 days shorter with teriparatide. The benefit was more pronounced in lower limb fractures (6.24 days shorter) than upper limb fractures. Functional recovery scores also improved.

These are not large differences. Other systematic reviews have found mixed results, with some trials showing no significant acceleration. The challenge is that fracture healing is harder to measure than BMD or fracture incidence, and studies have used inconsistent endpoints. Whether teriparatide meaningfully accelerates healing in clinical practice remains an open question.

What Happens After Treatment Stops?

Bone gained on teriparatide can be lost quickly after discontinuation. This is one of the drug's most important practical limitations.

The DATA-Switch study (Leder 2015) tracked what happens when patients switch between teriparatide and denosumab.^[6] After 24 months of teriparatide, women who transitioned to denosumab continued to gain BMD at all sites. The 4-year cumulative increase was substantial: spine and hip density rose progressively across both treatment phases.

The reverse sequence was problematic. Women who took denosumab first and then switched to teriparatide experienced transient bone loss at the hip and spine during the first year of teriparatide. BMD eventually recovered, but the dip was clinically meaningful.

The practical takeaway from DATA-Switch: when using these drugs sequentially, teriparatide should come first, followed by an antiresorptive to consolidate gains. Starting teriparatide after denosumab creates a temporary vulnerability window. This sequencing principle now appears in major treatment guidelines.

Women who stopped teriparatide and received no follow-up therapy lost BMD rapidly: femoral neck dropped 4.2%, total hip dropped 4.5%, and spine dropped 10.0%. Antiresorptive follow-up is not optional. It is essential to preserving what teriparatide builds.

The Osteosarcoma Question: From Black Box Warning to All Clear

In preclinical testing, rats given high-dose teriparatide for nearly their entire lifespan developed osteosarcoma (bone cancer) at elevated rates. The doses were far above human therapeutic levels and the exposure duration was far longer, but the finding was serious enough that the FDA required a black box warning at approval in 2002 and initially limited treatment to 24 months.

Over the next 18 years, a massive surveillance effort tracked the question in humans. Krege and colleagues published the comprehensive review in 2022.^[5]

Key findings from the surveillance program:

• More than 75,000 teriparatide-treated patients were tracked through pharmacy-cancer registry data linkage over 15 years
• No increase in osteosarcoma incidence was found compared to unexposed populations
• Incidence matched the expected background rate in the general population
• A separate primate study found no osteosarcoma signal in monkeys treated with teriparatide

Based on this totality of evidence, the FDA removed the black box warning in 2020 and revised the 24-month lifetime limitation. The osteosarcoma risk, at least at human therapeutic doses, appears to be a rat-specific phenomenon related to the extreme sensitivity of rodent bones to continuous PTH stimulation.

This does not mean the risk question is fully settled. The surveillance data are observational, not randomized, and osteosarcoma is extremely rare (about 900 cases per year in the US), making small increases in risk hard to detect. But the signal that prompted the original warning has not materialized in nearly two decades of monitoring.

How Teriparatide Compares to Newer Bone-Building Peptides

Teriparatide was the first anabolic osteoporosis drug, but it is no longer the only one. Two newer agents have entered the field.

Abaloparatide (Tymlos, approved 2017) is a synthetic analog of parathyroid hormone-related protein (PTHrP) rather than PTH itself. It binds the same PTH1R receptor but preferentially activates a specific receptor conformation (the RG conformation) that produces a shorter signaling duration. In clinical trials, abaloparatide produced similar vertebral fracture reductions with potentially greater nonvertebral fracture protection and less hypercalcemia than teriparatide. A 2023 meta-analysis found abaloparatide produced comparable or slightly greater BMD gains at the hip.^[3] The full head-to-head comparison of teriparatide and abaloparatide explores these differences in detail.

Romosozumab (Evenity, approved 2019) works through an entirely different mechanism: it is a monoclonal antibody that blocks sclerostin, a protein that inhibits bone formation. Romosozumab both increases formation and decreases resorption simultaneously, producing the fastest BMD gains of any osteoporosis drug. Recent data show romosozumab increased spine bone strength by approximately 40% after one year, outperforming teriparatide. However, romosozumab carries its own safety concern: a cardiovascular signal that led to its own black box warning, limiting its use in patients with recent heart attack or stroke.

Calcitonin, a peptide hormone from the thyroid gland, also affects bone metabolism but through an antiresorptive mechanism rather than an anabolic one.

Cost remains a differentiator. A full course of romosozumab costs approximately $22,000, abaloparatide approximately $48,000, and teriparatide approximately $86,000, though biosimilar entry (Bonsity) is beginning to reduce the teriparatide price. For patients with bone density concerns related to other peptide therapies, research on GLP-1 drugs and bone density is tracking whether rapid weight loss from incretin drugs affects skeletal health.

Side Effects and Practical Considerations

The most common side effects in the Neer 2001 trial were nausea (8.5% vs 6.7% placebo), dizziness (8.0% vs 5.4% placebo), and leg cramps (2.6% vs 1.3% placebo).^[1] Transient hypercalcemia occurs in some patients, particularly at higher doses.

Orthostatic hypotension can occur after the first few injections. This typically resolves within a few doses and does not require discontinuation.

Injection site reactions are generally mild. Because teriparatide requires daily self-injection, adherence can be challenging compared to weekly or monthly bisphosphonate pills. Real-world adherence studies consistently show lower persistence with injectable osteoporosis therapies compared to oral ones, though patients who stay on teriparatide tend to achieve BMD gains consistent with clinical trial results.

The Bottom Line

Teriparatide remains the most extensively studied anabolic peptide for osteoporosis, with two decades of clinical data supporting its ability to reduce fractures and build bone. The 65% vertebral fracture reduction from the Neer trial, the head-to-head superiority over bisphosphonates in VERO, and the resolution of the osteosarcoma safety question collectively make a strong evidence case. Its primary limitations are cost, the requirement for daily injection, and the critical need for follow-up antiresorptive therapy to maintain BMD gains after treatment ends.

Frequently Asked Questions

How does teriparatide work to build bone?

Teriparatide is a 34-amino-acid fragment of parathyroid hormone that, when given as a daily injection, stimulates osteoblasts to form new bone faster than it is resorbed. It activates the PTH1 receptor on bone-forming cells, triggering cAMP/protein kinase A signaling cascades that increase osteoblast number and survival. The key is intermittent dosing: continuous PTH exposure causes bone loss, but brief daily pulses shift the balance toward formation. The Neer 2001 trial showed this mechanism produced a 9-percentage-point increase in lumbar spine BMD over 21 months.^[1]

How much does Forteo reduce fracture risk?

In the pivotal 2001 trial of 1,637 postmenopausal women, teriparatide 20 mcg/day reduced new vertebral fractures by 65% (5% vs 14% placebo) and nonvertebral fractures by 53% (3% vs 6% placebo) over a median of 21 months.^[1] In the 2018 VERO trial comparing teriparatide directly to risedronate, vertebral fracture risk was 56% lower with teriparatide.^[4]

Does teriparatide cause bone cancer?

No increase in osteosarcoma has been found in humans. The original concern came from rat studies using extremely high doses for nearly the animals' full lifespan. A 15-year US surveillance program tracking over 75,000 teriparatide-treated patients found no elevated osteosarcoma risk compared to the general population.^[5] The FDA removed the black box warning in 2020 based on this evidence.

What happens when you stop taking teriparatide?

Bone density can decline rapidly after discontinuation if no follow-up treatment is given. In the DATA-Switch study, women who stopped teriparatide without antiresorptive follow-up lost 4.2% at the femoral neck, 4.5% at the total hip, and 10.0% at the spine.^[6] Transitioning to a bisphosphonate or denosumab after teriparatide preserves and can further increase BMD gains.

Is teriparatide better than bisphosphonates for osteoporosis?

In patients at high fracture risk, head-to-head trials favor teriparatide. The VERO trial showed 56% fewer vertebral fractures with teriparatide versus risedronate in women with severe osteoporosis.^[4] In glucocorticoid-induced osteoporosis, Saag 2007 found teriparatide produced double the spine BMD gain of alendronate with 90% fewer vertebral fractures.^[2] However, bisphosphonates are far less expensive and do not require daily injections, so they remain first-line for many patients at moderate risk.

How long can you take teriparatide?

The original FDA label limited treatment to 24 months based on preclinical osteosarcoma concerns in rats. In 2020, after a 15-year surveillance study found no cancer signal in humans, the FDA removed the black box warning and revised the lifetime limitation.^[5] Most treatment protocols still use 18-24 months of teriparatide followed by transition to an antiresorptive agent, as this sequence has the strongest evidence for sustained benefit.