Crysvita is a monoclonal antibody that targets and blocks the activity of a blood protein called FGF23. In a genetic condition called X-linked hypophosphatemia (HYE-poe-fos-fa-TEEM-ee-a), low phosphate levels in blood are caused by abnormally high levels of FGF23 protein, which causes the kidneys to stop reabsorbing phosphate into the bloodstream.

Blocking the FGF23 protein allows the kidneys to restore and maintain normal phosphate levels. Phosphate is important for the strength of your bones and teeth. Low phosphate levels can lead to bone deformities and growth problems.

Crysvita is used to normalize phosphate levels in adults and children who are at least 1 year old and have X-linked hypophosphatemia.

Crysvita may also be used for purposes not listed in this medication guide.

You should not use Crysvita if you have severe kidney disease, or if you currently take phosphate or vitamin D by mouth.

Your doctor will perform blood tests to make sure you do not have conditions that would prevent you from safely using Crysvita.

At least 1 week before your first injection, stop using oral phosphate and vitamin D medications.

Crysvita is injected under the skin, once every 2 weeks in children and once every 4 weeks in adults.

A healthcare provider will give you this injection.

Crysvita doses are based on weight in children. Your child's dose needs may change if the child gains or loses weight.

You may need frequent medical tests. Even if you have no symptoms, tests can help your doctor determine if this medicine is effective.

You should not use Crysvita if you are allergic to it, or if you have:

• severe or end-stage kidney disease; or

• if you currently take phosphate or vitamin D by mouth.

Tell your doctor if you have ever had:

• kidney disease; or

• restless leg syndrome.

It is not known whether Crysvita will harm an unborn baby. Tell your doctor if you are pregnant or if you become pregnant while using Crysvita.

It may not be safe to breast-feed while using this medicine. Ask your doctor about any risk.

Crysvita is not approved for use by anyone younger than 1 year old.

Call your doctor for instructions if you miss an appointment for your Crysvita.

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.

Follow your doctor's instructions about any restrictions on food, beverages, or activity.

Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Common side effects of Crysvita may include:

• headache, dizziness;

• pain in your arms, legs, or back;

• vomiting, constipation;

• a restless feeling in your legs;

• fever;

• tooth infection;

• decreased vitamin D levels;

• increased phosphorous levels; or

• pain, redness, itching, swelling, bruising, or a hard lump where the medicine was injected.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Other drugs may affect Crysvita, including prescription and over-the-counter medicines, vitamins, and herbal products. Tell your doctor about all your current medicines and any medicine you start or stop using.