Natalizumab is used to treat multiple sclerosis-MS. It is not a cure for MS, but it is thought to help by preventing your immune system from attacking the nerves in your brain and spinal cord. It helps decrease the number of episodes of worsening symptoms and may prevent or delay disability. Natalizumab is also used to treat a bowel condition called Crohn's disease (CD). It is not a cure for CD, but it is thought to work by preventing your immune system from causing inflammation/swelling within your bowels. Natalizumab is a protein called a monoclonal antibody.

Read the Medication Guide provided by your pharmacist before you start using natalizumab and each time you receive another dose. If you have any questions, ask your doctor or pharmacist.

This medication is given by injection into a vein by a health care professional as directed by your doctor, usually every 4 weeks. This medication is usually given over 1 hour.

Infusion reactions may happen during the infusion of this drug and for a short time afterward. Tell your doctor right away if you have any symptoms of infusion reactions such as chills, fever, flushing, nausea, dizziness, tiredness, or chest pain.

It is important to use this medication regularly to get the most benefit from it. Do not miss any doses without your doctor's approval.

Tell your doctor if your condition gets worse. When using this medication for Crohn's disease, if your condition does not get better after 12 weeks of treatment, your doctor will need to switch your treatment plan.

See also How to Use section.

Headache, joint pain, redness/irritation at the injection site, swelling hands/feet/ankles, or changes in menstrual cycle may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly.

Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

Tell your doctor right away if you have any serious side effects, including:

• headache that is severe or doesn't go away
• stiff/painful neck
• fast/pounding heartbeat
• easy bruising/bleeding
• signs of infection (such as sore throat that doesn't go away, fever, breathing problems, unusual vaginal discharge, painful/frequent urination)
• mood changes (such as depression, suicidal thoughts)
• severe stomach/abdominal pain

This drug increases the risk of a rare, possibly fatal, brain infection (see Warning section for more details). This condition may occur during treatment or, in some cases, after treatment has stopped. In MS patients, the symptoms of PML can seem like an attack of worsening MS. Whether you are using this drug or have stopped using it within the last 6 months, tell your doctor right away of any new or worsening symptoms that have lasted for several days such as:

• clumsiness
• sudden change in your thinking (such as confusion, difficulty concentrating)
• difficulty moving muscles
• seizure
• problems with speech
• vision changes

This drug may rarely cause serious liver problems. If you notice any of the following rare but very serious side effects, tell your doctor right away:

• nausea/vomiting that doesn't stop
• dark urine
• yellowing eyes/skin
• feeling tired/weak

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including:

• rash
• itching/swelling (especially of the face/tongue/throat)
• severe dizziness
• trouble breathing

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Before using this medication, tell your doctor or pharmacist if you are allergic to it; or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details.

Before using this medication, tell your doctor or pharmacist your medical history, especially of:

• a certain virus infection (progressive multifocal leukoencephalopathy-PML)
• history of certain virus infections that keep coming back (such as herpes, shingles)
• weakened immune system (such as leukemia, lymphoma, HIV infection, organ transplant)
• current infections
• mental/mood disorders (such as depression)

Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products).

During pregnancy, this medication should be used only when clearly needed. Babies born to mothers who have used this drug during pregnancy may have low platelet and red blood cell counts. Tell the doctor right away if your baby develops easy bruising/bleeding. Discuss the risks and benefits with your doctor.

It is unknown if this drug passes into breast milk. Because of the possible risk to the infant, breastfeeding is not recommended while using this drug. Consult your doctor before breastfeeding.

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval.

Some products that may interact with this drug include:

• past or current use of other drugs that weaken the immune system/increase the risk of infection (such as azathioprine, cyclosporine, 6-mercaptopurine, methotrexate, fingolimod, TNF blockers such as adalimumab, etanercept, infliximab)
• long-term use of corticosteroids (such as dexamethasone, prednisone)

If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call 1-800-222-1222. Canada residents can call 1-844-764-7669.

Lab and/or medical tests (such as MRI, liver function, anti-JCV antibody test) may be done before you start using this medication and while you are using it. Keep all medical and lab appointments. Consult your doctor for more details.

Talk to your doctor or pharmacist about lifestyle changes that might benefit you. Examples of lifestyle changes include stress reduction programs and maintaining a healthy diet. A doctor-approved exercise program may also help MS patients maintain strength, balance, and muscle tone. Consult your doctor or pharmacist for more details.

It is important to get each dose of this medication as scheduled. If you miss a dose, ask your doctor or pharmacist right away for a new dosing schedule.

Not applicable. This medication is given in an infusion center and will not be stored at home.

Your condition can cause complications in a medical emergency. For information about enrolling in MedicAlert, call 1-888-633-4298 (US) or 1-800-668-1507 (Canada).