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Dalfampridine is a potassium-channel antagonist believed to improve transmission of action potentials in demyelinated axons. In this 9-week, multicenter, double-blind, manufacturer-sponsored trial, investigators randomized 239 patients with all subtypes of multiple sclerosis (MS) to dalfampridine (10 mg twice daily) or placebo. A positive response was predefined as improved speed on the Timed 25-Foot Walk on at least three of four treatment assessments (conducted every 2 weeks), compared with the maximum speed obtained from any of five baseline visits. All subjects were on placebo for two of these five baseline visits as part of a subject-blinded placebo run-in before randomization. This stringent responder outcome measure was approved for …