Natalizumab (Tysabri) is a drug treatment for people with relapsing forms of MS. It's approved as a monotherapy, which means it's not to be taken in combination with any other disease-modifying drug. Studies show that natalizumab is associated with a reduced risk of relapses, lesions and disability progression.

Natalizumab is associated with some risks and side effects, including:

• Fatigue
• Anxiety
• Infections
• Sore throat
• Sinus congestion
• Swelling
• Liver toxicity
• Allergic reactions

Liver complications are possible during the first few months of treatment, but they normally go away.

Natalizumab rarely can be associated with a serious condition known as progressive multifocal leukoencephalopathy (PML). PML is a central nervous system disorder caused by infection with JC virus (JCV). This virus infects 60% to 80% of the world's population without consequences but can cause PML in people who have immunocompromised conditions.

Because natalizumab increases the risk of PML, your doctor should consider whether the expected benefits of taking it are sufficient to offset this risk. The main PML risk factors include:

• Presence of anti-JCV antibodies
• Prior use of immunosuppressant medications
• Treatment with natalizumab for more than two years

Based on the risk factors, treatment recommendations have been proposed that include testing for JCV antibodies prior to starting treatment with natalizumab, every three to six months during treatment, and for at least six months after discontinuing treatment.