Intravenous steroids are safe and effective in treating acute exacerbations of MS. Its use is directed at the early halting or diminishing of the destructive inflammatory process in the central nervous system, so that neurologic disability doesn't accumulate. For an acute relapse, a course of intravenous corticosteroids is typically given (500 mg to 1 gram of methylprednisolone (Solu-Medrol) over 30 to 60 mins for 3 days). This course can be extended up to 5 days (or to even 10 days) if the attack continues to progress or is slow in improving. Intravenous methylprednisolone is also the usual primary treatment for optic neuritis. The somewhat rapid effect of steroid treatment is based partly by reduction of white matter edema, and somewhat by an alteration of immunological factors. It is unusual in practice to give more than 2 or 3 courses of steroids for the treatment of relapses.