Natalizumab (Tysabri) is a treatment for people with relapsing forms of multiple sclerosis (MS). It makes flares happen less often and keeps physical disabilities from getting worse quickly.
Tysabri works in a different way from other MS drugs. It keeps white blood cells from entering your brain and spinal cord, which doctors think plays an important role in the damaging effects of MS.
Your risk for a rare brain infection goes up with the number of doses you take and if you've taken drugs that dampen your immune system before you use it. So doctors usually recommend Tysabri only for people who've tried other treatments for multiple sclerosis that haven't worked.