Forteo is Eli Lilly's brand of teriparatide, the first 34 amino acids of human parathyroid hormone, approved in the US in November 2002 as the first bone-building osteoporosis drug. It is injected once a day. Its closest relative is abaloparatide (Tymlos), whose trial gave both molecules side by side — that comparison is on our Tymlos side effects page.
This page reports what Forteo's label and its studies recorded, with dates. Dose units, pens and the duration wording are on the teriparatide dosage page; Peptide Lexicon explains the molecule in its teriparatide entry. Nothing here is a recommendation; a side effect that worries a patient is a question for the prescriber.
The placebo table
The label pools two randomised, double-blind trials: postmenopausal women (the trial published by Neer and colleagues in 2001, 1,637 women at 20 or 40 mcg against placebo) and men with osteoporosis. For the 20 mcg dose, 691 people received teriparatide and 691 placebo; median treatment was 19 months for women and 11 months for men. The table lists events in at least 2% of Forteo users and more often than on placebo, "without attribution of causality".
| Event | Forteo (n = 691) | Placebo (n = 691) | Difference |
|---|---|---|---|
| Pain | 21.3% | 20.5% | +0.8 |
| Arthralgia | 10.1% | 8.4% | +1.7 |
| Rhinitis | 9.6% | 8.8% | +0.8 |
| Asthenia | 8.7% | 6.8% | +1.9 |
| Nausea | 8.5% | 6.7% | +1.8 |
| Dizziness | 8.0% | 5.4% | +2.6 |
| Headache | 7.5% | 7.4% | +0.1 |
| Hypertension | 7.1% | 6.8% | +0.3 |
| Insomnia | 4.3% | 3.6% | +0.7 |
| Depression | 4.1% | 2.7% | +1.4 |
| Vertigo | 3.8% | 2.7% | +1.1 |
| Syncope (fainting) | 2.6% | 1.4% | +1.2 |
| Leg cramps | 2.6% | 1.3% | +1.3 |
| Angina pectoris | 2.5% | 1.6% | +0.9 |
Forteo label effective August 2026, Table 1, via openFDA (read 27 October 2026); the difference column is our arithmetic in percentage points. No single event is more than 2.6 points above placebo, and serious adverse events were less frequent on Forteo (16%) than on placebo (19%). The 2001 trial paper described the side effects as "only minor", naming occasional nausea and headache.
In a separate 18-month trial in glucocorticoid-induced osteoporosis, Forteo was compared with an oral bisphosphonate rather than placebo (214 patients each). Events at least 2 points more frequent on Forteo: nausea 14% vs 7%, gastritis 7% vs 3%, pneumonia 6% vs 3%, shortness of breath 6% vs 3%, insomnia 5% vs 1%, anxiety 4% vs 1% and shingles 3% vs 1%.
Calcium: the one effect with a clock
Teriparatide raises blood calcium for a few hours after every dose. The label puts the peak at four to six hours and the level back to pre-treatment by 16 hours. In the trials:
| Women | Men | |
|---|---|---|
| At least one high calcium reading 4–6 h after a dose, Forteo | 11% | 6% |
| Same, placebo | 2% | 0% |
| Confirmed on consecutive measurements, Forteo | 3% | 1% |
Urine calcium rose, but hypercalciuria was no more frequent than on placebo; serum uric acid was above normal in 3% vs 1%, without more gout or kidney stones. After marketing, calcium above 13 mg/dL has been reported, and the label now carries a warning on calciphylaxis and worsening of previously stable skin calcification — reported with risk factors including autoimmune disease, kidney failure, and warfarin or systemic corticosteroid use. Because calcium predisposes to digitalis toxicity, the label also flags use with digoxin.
Dizziness, fainting and the first doses
The syncope and dizziness rows in the table have a label explanation. In short-term studies in healthy volunteers, 5% had transient symptomatic orthostatic hypotension: it began within four hours of dosing, happened within the first several doses, resolved within minutes to hours, was relieved by lying down, and did not stop treatment. The label instructs that the first doses be given where the person can sit or lie down.
Forteo against abaloparatide, in the same trial
The ACTIVE trial randomised 818 women to open-label teriparatide alongside blinded abaloparatide and placebo. Its posted results give teriparatide's rates beside the newer drug: palpitations 1.6% (abaloparatide 5.1%, placebo 0.4%), nausea 5.1% (8.3%, 3.0%), dizziness 7.2% (10.0%, 6.1%), hypercalciuria 12.5% (11.3%, 9.0%) (ClinicalTrials.gov NCT01343004, read 27 October 2026; our percentages from posted counts). The trial's prespecified hypercalcaemia endpoint was 6.4% on teriparatide against 3.4% on abaloparatide. Because teriparatide was open-label, its participants knew what they were taking; the full table and its limits are on the Tymlos page.
The osteosarcoma question, in numbers
The 2002 label carried a boxed warning because teriparatide caused osteosarcoma in rats at exposures 3 to 60 times the human dose, depending on dose and on how long and from what age the rats were treated. It also said use beyond two years "is not recommended".
| Label version (Drugs@FDA, NDA 021318) | Date | Osteosarcoma | Duration |
|---|---|---|---|
| Original approval | November 2002 | Boxed warning | "use of the drug for more than 2 years is not recommended" |
| Supplement 053 | April 2020 | Boxed warning, unchanged | Unchanged |
| Supplement 054 | November 2020 | "Osteosarcoma Boxed Warning, Removed" | Beyond 2 years "should only be considered if a patient remains at or has returned to having a high risk for fracture" |
| Supplement 059 (current) | August 2026 | Warning section; hypersensitivity warning added 08/2026 | Unchanged since 2020 |
What the human record shows, as the current label states it: two US claims-database surveillance studies found three osteosarcoma cases among 379,283 Forteo users and none among 153,316 — 532,599 users in all. The label says the results "suggest a similar risk" to comparators, and immediately cautions that the data sources do not allow complete control for confounders. Cases of bone tumour and osteosarcoma "have been reported rarely" after marketing. The warning section keeps a list of people at higher baseline risk — open growth plates, Paget's disease, bone metastases or skeletal cancer, prior skeletal radiation, hereditary predisposition.
Tymlos's boxed warning came off thirteen months later, in December 2021, but its label kept the two-year "not recommended" wording that Forteo's lost.
Allergic reactions and other post-marketing reports
The August 2026 label adds a hypersensitivity warning: anaphylaxis, angioedema and hives have been reported with PTH analogues, and severe hypersensitivity to teriparatide is now a contraindication. Other reactions reported since approval, with frequency unknown: acute shortness of breath, chest pain, leg or back muscle spasms, swelling of the face and mouth, and injection-site pain, swelling and bruising. How these compare with other peptide injections is on our injection-site reactions page.
Generics and follow-on products
Drugs@FDA lists, as of 27 October 2026, Forteo (one pen strength still marketed, one discontinued), Bonsity (Alvogen, approved 2019), a teriparatide NDA from Almaject (2024), and generics from Apotex and Teva (2023) and Amphastar (2025). Their labels carry the same warnings; their side-effect data are Forteo's.
What is not established
- Osteosarcoma risk beyond two years of use, which the label calls limited.
- Whether the surveillance studies would detect a small increase in a cancer this rare, given the confounding the label itself flags.
- How long individual side effects last, beyond the calcium time course and the first-dose blood pressure effect.
- A blinded head-to-head comparison with abaloparatide — the only one is open-label.
Anyone with a side effect they are unsure about should take it to the prescriber or pharmacist; reports go to the manufacturer or to FDA's MedWatch.
Sources and dates
- Forteo (teriparatide) prescribing information, label effective August 2026, via openFDA drug label API — read 27 October 2026.
- Drugs@FDA, NDA 021318: label PDFs for the original approval (11/2002), supplements 053 (04/2020), 054 (11/2020) and 059 (08/2026); teriparatide applications by sponsor — read 27 October 2026.
- ClinicalTrials.gov API v2, NCT01343004 (ACTIVE), posted adverse events — read 27 October 2026.
- Neer RM et al. Effect of parathyroid hormone (1-34) on fractures and bone mineral density in postmenopausal women with osteoporosis. N Engl J Med. 2001;344(19):1434–41. PMID 11346808.
- Miller PD et al. Effect of abaloparatide vs placebo on new vertebral fractures. JAMA. 2016;316(7):722–33. PMID 27533157.