Tizanidine is used to relieve the spasms and increased muscle tone caused by multiple sclerosis (MS, a disease in which the nerves do not function properly and patients may experience weakness, numbness, loss of muscle coordination and problems with vision, speech, and bladder control), stroke, or brain or spinal injury. Tizanidine is in a class of medications called skeletal muscle relaxants. It works by slowing action in the brain and nervous system to allow the muscles to relax.
Summary from MedlinePlus.gov, click for more information.