This is a parallel-group, double-blind, randomized controlled crossover trial with a 1:1 randomization. There are two treatment periods of 4 weeks and a two-week off treatment washout period. Treatment periods are DER (dalfampridine ER) followed by placebo (group A) and placebo followed by DER (group B), respectively. All dosages will be administered twice a week. Purpose of the study is to see if using the study drug(dalfampridine Er) for a short period of time will improve the way subjects with multiple sclerosis (MS) walk.
