Cancer has not been reported as a side effect of Tysabri (natalizumab) in clinical studies. But you should tell your doctor if you have cancer now or a history of cancer in the past before you start treatment with Tysabri.

Cancer and cancer treatments can weaken your immune system, which can make treatment with Tysabri dangerous due to a rare brain infection known as progressive multifocal leukoencephalopathy (PML).

Tell your doctor if you have any health conditions that may weaken your immune system such as:

• HIV infection or AIDS
• leukemia or lymphoma
• an organ transplant
• other medical conditions that can weaken your immune system

Treatment with Tysabri can weaken your immune system. Tysabri can increase your chances of getting a rare brain infection called progressive multifocal leukoencephalopathy (PML). PML usually happens in people with a weakened immune system and often leads to death or severe disability.

There is no way to treat, prevent, or cure PML at this time. Your doctor can give you a blood test to see if you are at higher risk for PML. If you have or have had PML, you cannot use Tysabri.

Other serious side effects that may occur if you take Tysabri include:

• herpes infections of the brain or eyes
• liver damage
• allergic reactions, including a possible life-threatening reaction called anaphylaxis
• unusual or serious infection
• low platelet counts

The most common side effects of Tysabri include: headache, feeling tired, urinary tract infection, joint pain, lung infection, depression, pain in your arm and legs, diarrhea, vaginitis, rash, nose and throat infections, nausea, and stomach area pain.

Tell your doctor about all of the medicines you are taking, including prescription medicines, over-the-counter (OTC) drugs, herbal supplements and vitamins. Be sure to tell your doctor if you take medicines that weaken your immune system, and ask your doctor if you are not sure.

Why is Tysabri prescribed?

Tysabri is a prescription medicine used in adults 18 years of age and older to treat multiple sclerosis or Crohn’s disease. It is given as an injection.

• It is used to treat relapsing forms of multiple sclerosis in adults (including clinically isolated syndrome, relapsing-remitting disease, and active secondary progressive disease).
• It is also used to treat moderate to severe Crohn's disease in adults when other medicines did not work or have stopped working.

A higher risk of certain cancers of the digestive tract (colon) have been reported in patients with Crohn’s disease and other inflammatory bowel diseases.

Before using this medicine, you should discuss and understand the risks and benefits of Tysabri with your doctor. You will need to follow a specialized prescribing program to have access to this medicine and to help manage your safety. Your doctor can explain this program to you and what your requirements are.

This is not all the information you need to know about Tysabri (natalizumab) for safe and effective use and does not take the place of talking to your doctor about your treatment. Review the full Tysabri information here, and discuss this information and any questions you have with your doctor or other health care provider.