Teriparatide (Forteo): How It Works, Dosage, Side Effects & Cost

Teriparatide builds new bone.

It's the first 34 amino acids of human parathyroid hormone, made by recombinant DNA, and the only osteoporosis drug class that actively grows new bone instead of just slowing the loss of old bone. Sold as Forteo (Eli Lilly) and the generic Bonsity, it's prescribed for postmenopausal women, men with primary or hypogonadal osteoporosis, and people with steroid-induced bone loss who are at high risk for fractures. Below is exactly how teriparatide works, the once-daily 20 mcg pen protocol, the side effects worth knowing, the cost reality, and how it compares to abaloparatide and bisphosphonates.

🔑 Key Takeaways

What Is Teriparatide?

Teriparatide is the active 34-amino-acid N-terminal fragment of human parathyroid hormone (PTH), made by recombinant DNA technology in E. coli. The full natural PTH molecule has 84 amino acids; teriparatide is just the part that does the bone-building work, which is why one of its common names in clinical literature is simply "PTH 1-34."

It was first FDA-approved in 2002 under the brand name Forteo (Eli Lilly), then a generic teriparatide injection was approved in 2019 under the name Bonsity. In 2023, the EU approved an additional biosimilar called Kauliv. The active drug is the same across all three: same molecule, same 20 mcg daily dose, same prefilled pen format.

How Teriparatide Works

This is the part that surprises people: parathyroid hormone is normally what your body uses to break down bone. Chronically high PTH causes osteoporosis. So how does injecting a piece of PTH build bone?

The answer is timing. The two opposite effects of PTH come from how long the signal lasts.

1. Continuous PTH exposure (the kind you'd see in hyperparathyroidism) activates osteoclasts more than osteoblasts. Net result: bone resorption, calcium pulled from bone into blood, weaker skeleton.
2. Intermittent PTH exposure (one short pulse a day, the way teriparatide is dosed) activates osteoblasts more than osteoclasts. Net result: new bone formation, especially in the lumbar spine and femoral neck.

That's why teriparatide is given exactly once per day, every day, by subcutaneous injection. The subcutaneous half-life is about 1 hour, which keeps the PTH pulse brief and bone-building. If teriparatide were given as a continuous infusion or multiple times daily, it would start acting like endogenous hyperparathyroidism and erode bone instead.

Clinically, the result over 18 to 24 months of daily use is:

• Bone mineral density (BMD) increases of 9-13% at the lumbar spine
• BMD increases of 3-6% at the femoral neck and total hip
• About a 65% reduction in new vertebral fractures and a 53% reduction in nonvertebral fractures in postmenopausal women
• Improvements in bone microarchitecture (trabecular thickness, connectivity), not just density

FDA-Approved Uses

Teriparatide is approved for three specific populations, all defined by high fracture risk:

It is not approved for, and should not be used for, general "anti-aging" bone support, athletic recovery, or cosmetic indications. The risk-benefit profile only makes sense in genuine high-fracture-risk osteoporosis.

Teriparatide Dosage and How to Inject

Step-by-Step Pen Use

1. Take the pen out of the refrigerator and let it warm slightly. Don't microwave or run under hot water.
2. Check the liquid. It should be clear and colorless. Do not use if cloudy, colored, or has particles.
3. Wash hands and swab the injection site (abdomen or thigh) with isopropyl alcohol.
4. Attach a new needle each time. Never store the pen with the needle attached.
5. Set the dose per the pen's instructions, pinch the skin, and inject at a 90° angle.
6. Hold for 6 seconds after pressing the button, then remove and discard the needle in a sharps container.
7. Recap and refrigerate the pen immediately. Don't leave it out.

If You Miss a Dose

Take it as soon as you remember, on the same calendar day. If it's already the next day, skip the missed dose, don't double up. Continue your normal once-daily schedule. Missing one dose won't undo your progress, but consistency matters over months.

Teriparatide Side Effects

Most teriparatide side effects are mild and concentrated in the first weeks. The serious ones are rare but worth knowing because they shape who can safely use the drug.

Common Side Effects

• Nausea (about 9% of users)
• Joint pain and arthralgia (about 10%)
• Leg cramps (about 3%)
• Dizziness
• Headache
• Injection site reactions: mild redness, bruising, or itching
• Brief calcium increase after each dose (usually well within safe range)

Serious Side Effects

• Orthostatic hypotension within 4 hours of the first few doses. Sit or lie down for early doses.
• Hypercalcemia (persistent high blood calcium). Symptoms: nausea, vomiting, constipation, muscle weakness, confusion. Stop the drug and call your prescriber if it persists.
• Kidney stones from sustained high calcium output.
• Allergic reactions including angioedema or anaphylaxis. Rare but possible with any injectable protein drug.

The Osteosarcoma Question

Teriparatide's original FDA label carried a black-box warning for osteosarcoma (bone cancer) because high-dose toxicology testing in young, still-growing animal models showed an increased bone tumor rate at 3-60 times the human exposure level. That signal was always considered a worst-case extrapolation because the test subjects had open growth plates that don't exist in adult humans.

The long-term human surveillance study that followed told a different story. Over 75,000 patients tracked from 2009 to 2019 produced no cases of osteosarcoma attributable to teriparatide. The FDA acted on that data in 2020 and removed the strict 2-year lifetime cumulative use limit. The warning about avoiding the drug in patients with elevated baseline osteosarcoma risk (prior skeletal radiation, Paget's disease, open growth plates, unexplained alkaline phosphatase elevation, or bone metastases) remains, but the implied "two years and you're out" rule no longer applies.

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Who Should Not Use Teriparatide

Teriparatide is contraindicated or strongly cautioned in:

• Patients with hypercalcemia at baseline
• People with active or prior bone cancers, bone metastases, or skeletal radiation history
• Paget's disease of bone (raises osteosarcoma baseline risk)
• Anyone with open epiphyses (children, adolescents whose bones are still growing)
• Severe kidney impairment
• Untreated hyperparathyroidism
• Pregnant or breastfeeding women
• Anyone with a known hypersensitivity to teriparatide or pen excipients

Storage and Pen Handling

Storage rules are strict and breaking them ruins the drug.

• Always refrigerate at 36-46°F (2-8°C). Don't freeze. Freezing destroys the protein.
• Protect from light. Keep the pen in its outer carton when stored.
• After first use, the pen is good for 28 days. Throw it away on day 29 even if there's medication left.
• Don't store the pen with the needle attached. Always use a fresh needle each injection.
• Travel with a cooler bag on day trips. For longer travel, plan for refrigeration on both ends.

Cost: Forteo, Bonsity, and the Savings Card Reality

Teriparatide's biggest practical barrier is sticker price.

If you're on Medicare or any government drug program, the Lilly $4 savings card does not apply, and your out-of-pocket cost depends on your specific plan's specialty drug tier. Many Medicare Part D plans require prior authorization plus a fracture history before they cover teriparatide. Talk to your prescriber and pharmacist about Bonsity or other generic teriparatide if cost is the limiting factor, the molecule is identical.

Teriparatide vs Abaloparatide vs Romosozumab vs Bisphosphonates

Teriparatide isn't the only option for severe osteoporosis. Here's how it stacks up against the modern alternatives.

For most patients with high but not extreme fracture risk, oral or IV bisphosphonates are tried first because they're cheap and have decades of evidence. Teriparatide enters the picture when bone density is severely low, fractures have already happened, or bisphosphonates failed or weren't tolerated. Romosozumab is often used in the same severe-risk patients as teriparatide; some specialists prefer it for cardiovascular-low-risk patients because the bone gains are larger and the course is shorter.

What Happens After You Stop Teriparatide

This is the part patients aren't always warned about clearly. Teriparatide's bone gains begin reversing within months of stopping. The new bone you built was real, but without ongoing osteoblast stimulation and without an antiresorptive to keep osteoclasts in check, density slides back toward where it started.

The standard sequence is:

1. Teriparatide for 18-24 months to build new bone.
2. Transition immediately to a bisphosphonate (alendronate, zoledronic acid) or denosumab (Prolia) without a gap.
3. Maintain on the antiresorptive long-term to lock in the gains.

Skipping the follow-on therapy is one of the most common reasons teriparatide "doesn't work" in real-world practice. The drug works, the protocol just got cut off halfway through.

Frequently Asked Questions