Tysabri (natalizumab) is an IV med used to treat relapsing/remitting MS, prescribed to help reduce acute MS attacks and decrease or delay disability. Side effects of Tysabri include headache, joint pain, changes in menstrual cycle in women, and swelling. Tysabri has also been linked to a rare but potentially fatal brain infection called progressive multifocal leukoencephalopathy (PML). The risk of PML is higher if people have been on Tysabri for a long time or are taking it along with other immune-suppressing medications.