Disclosed herein is a method for treating patient suffering from a disorder related to central nervous system with an oral dosage formulation that contains both sustained-release and immediate-release drugs. The sustained-release drug in the oral dosage formulation is memantine, and the formulation provides an average blood level Cmax of the memantine in a range of about 10-24 ng/mL during an average Tmax of about 10-45 hours. The immediate-release drug in the oral dosage formulation is an acetylcholinesterase inhibitor (AChEI), and the formulation provides an average blood level Cmax of the AChEI in a range of about 12-38 ng/mL during an average Tmax of less than about 4 hours.