Tysabri (natalizumab) is a laboratory-produced monoclonal antibody used for treating multiple sclerosis (MS). It works by preventing potentially damaging immune cells in the bloodstream from crossing the blood-brain barrier into the brain and spinal cord. The FDA approved it in 2006 to treat relapsing forms of MS. Because of the risks associated with Tysabri, it’s approved only as a monotherapy, which means that it shouldn’t be used in combination with any of the other disease-modifying therapies.

The FDA also recommends that it only be offered to people who haven’t received adequate benefit from the first-line MS medications or have been unable to tolerate the injections or the side effects associated with those medications.

Tysabri has been shown to reduce the risk of disability progression as well as the number of relapses and the development of new or enlarging lesions on MRI scans. It’s delivered every four weeks by intravenous infusion (a slow drip through a needle inserted into a vein).The most common side effects include headache, fatigue, urinary tract infections, lower respiratory tract infections, joint pain, and chest discomfort.

Even though Tysabri has never been compared directly with the first-line treatment options, the clinical trial data suggest that it may well be more effective than the injectable medications in reducing relapses, lesions on MRI, and the risk of disability progression.

These benefits come with significantly greater risks. A rare and potentially fatal brain infection known as PML can occur in people taking Tysabri. The FDA has put procedures in place to maximize the safety of this medication, the major risk factors for developing the infection, and the blood test that can help identify those people at greatest risk for the infection.