If these drugs don't work for you, the next step might be to try a different disease-modifying medication called natalizumab (Tysabri). It prevents immune cells from getting to your brain and spinal cord, where they can damage nerves. The drug has been linked to progressive multifocal leukoencephalopathy (PML), so your doctor may suggest it only if other medications fail. If you do take it, your doctor will keep an eye on you carefully. A blood test can help doctors check risk for PML. In the most severe cases of multiple sclerosis that don't respond to medication, doctors may prescribe mitoxantrone (Novantrone) or alemtuzumab (Lemtrada). Both are chemotherapy drugs designed to treat cancer. They are used to treat certain types of MS by curbing the immune system to prevent it from attacking nerve coverings. Novantrone is used sparingly, though, because it has an FDA "black box" warning because of risks of heart damage and a type of leukemia.