A Placebo Controlled Trial of Dalfampridine ER for Ambulatory Activity in People With Multiple Sclerosis

Inclusion Criteria: * Diagnosis of MS made at least 3 months prior to based McDonald or Proser criteria. * Age 18-75 years old inclusive. * Expanded Disability Status Scale (EDS) 0-6.5 * Clinical stability defined as no MS exacerbation or change in disease modifying therapy for 60 days prior to screening. * Screening 6-minute walking test distance between 50m-500m, inclusive. * Written informed consent. Exclusion Criteria: * use of 4-aminopyridine within 6 months of screening * Any contraindication to DER: * Allergy to DER * history of seizure disorder or history of EEG showing epileptiform activity * Renal insufficiency (estimated GFR \< 60. * Any condition that would exclude 6 minute walking testing: * Cardiac surgery or myocardial infarction within the last 3 months. * Severe aortic stenosis or hypertropic cardiomyopathy. * Pulmonary embolus or infarction in the last 6 months. * Uncontrolled hypertension by history or by screening or baseline diastolic blood pressure \> 170, or systolic blood pressure \> 105. * Use of oxygen at home for 24 hours/day or severe lung disease. * History of ventricular arrhythmia or finding of significant ventricular arrhythmia. atrial arrhythmia or 2nd or 3rd degree heart block on screening ECG. * Concomitant neurological disease, such as ALS, Parkinson Disease, stroke. * Hospitalization in the last 6 months for psychiatric illness. * Alcohol or drug abuse within the past year. * Females who are breast-feeding, pregnant or have potential to become pregnant during the course of the study( fertile and unwilling/unable to use effective contraceptive measures) * Cognitive deficits that would interfere with the subject's ability to give informed consent or preform study testing. * Any other serious and/or unstable medical condition. * Use of mitoxantrone (Novantrone) within 6 months of baseline visit.