 The objectives of the study are to evaluate the safety, tolerability, and efficacy of 3 doses of AVP-923 dextromethorphan DMquinidine Qcapsules containing either 45 mg DM and 10 mg Q AVP-923-45 or 30 mg DM 10 mg Q AVP-923-30 or 20 mg DM and 10 mg Q AVP-923-20 compared to placebo, for the treatment of central neuropathic pain in a population of patients with multiple sclerosis MS over a 12-week period. The MS patients population enrolled includes patients with relapsing-remitting multiple sclerosis RRMS and patients with secondary progressive multiple sclerosis SPMS. 