Forteo Side Effects: What to Expect and When to Call Your Doctor

Forteo (teriparatide) is generally well tolerated, but like all medications, it can cause side effects. Knowing what to expect—and which symptoms require a call to your doctor—helps you stay safe and get the most from your treatment.

The Boxed Warning: Osteosarcoma Risk

Forteo carries a boxed warning—the FDA's most serious warning—about the risk of osteosarcoma (a malignant bone tumor). This is based on animal studies in rats where high doses of teriparatide caused osteosarcoma.

In people, this risk is considered extremely rare. The FDA estimates a rate of roughly 1 in 100,000 users, which is actually higher than the natural osteosarcoma rate in people over age 60 (0.4 in 100,000). Human studies have not confirmed an increased risk of osteosarcoma with teriparatide use. Nonetheless, Forteo is contraindicated in certain high-risk patients (those with Paget's disease, prior skeletal radiation, or existing bone cancers) and should not be used for more than 2 years.

Call your doctor right away if you notice bone pain that doesn't go away, unusual lumps or swelling under the skin, or any area of your body that is tender to the touch.

Common Side Effects (Reported in Clinical Trials)

The following side effects occurred more often in Forteo-treated patients than in control groups during clinical trials:

Nausea (14%): One of the most commonly reported side effects. Often mild and tends to improve after the first few weeks.

Orthostatic hypotension (dizziness when standing): Can occur, especially with the first few doses. Eli Lilly recommends the first injection be given while the patient is sitting or lying down.

Arthralgia (joint pain): Aching in the joints, reported commonly.

Leg cramps: Particularly in the calves; may be related to mild calcium shifts.

Gastritis (7%) and stomach discomfort: Inflammation of the stomach lining.

Insomnia (5%): Trouble sleeping reported in some patients.

Headache: Generally mild.

Injection site reactions: Mild redness, swelling, or bruising at the injection site. Rotate injection sites to minimize these.

Serious Side Effects: When to Contact Your Doctor

Some side effects require prompt medical attention:

Signs of high calcium (hypercalcemia): Forteo can temporarily raise serum calcium. Symptoms include nausea, vomiting, constipation, low energy, muscle weakness, or excessive thirst. Call your doctor if you experience these.

Severe dizziness or fainting: Significant orthostatic hypotension. If you feel faint after injection, lie down immediately and seek medical advice.

Allergic reactions: Rare but can include hives, difficulty breathing, or swelling of the face/throat. Seek emergency medical care immediately if this occurs.

Bone pain that won't resolve: As noted above, persistent bone pain or unusual lumps should be reported to your doctor promptly.

Tips to Reduce Side Effects

Give your first injection while sitting or lying down, and stay that way for at least 15 minutes afterward—this reduces the risk of dizziness

Rotate injection sites between your thigh and abdomen to reduce injection site reactions

Take Forteo at the same time each day—many patients prefer evening to sleep through initial dizziness or nausea

Stay well hydrated and maintain adequate dietary calcium and vitamin D as directed by your doctor

The Bottom Line

Most patients tolerate Forteo well. Common side effects like nausea and mild dizziness often improve over time. The osteosarcoma warning, while serious, reflects an extremely rare risk in humans and should be discussed with your doctor in the context of your overall bone health. For information on medications to avoid while taking Forteo, read our guide on Forteo drug interactions.