AIM To evaluate the efficacy and safety of memantine oral solution in the treatment of moderate to severe Alzheimer's disease(AD).METHODS A total of 237 patients with moderate to severe AD were randomly assigned to 2 groups received memantine oral solution 20 mL·d~(-1)or memantine tablet 20 mg·d~(-1) for 16 weeks respectively.The Alzheimer's Disease Assessment Scale-cognition(ADAS-Cog),the Activities of Daily Living(ADL)and Clinical Global Impression(CGI)were evaluated at baseline and 4,8,12 and 16 weeks after the treatment respectively to assess the efficacy.Laboratory testing and physical examination were performed at 8 and 16 weeks and the vital signs and adverse events were recorded to assess the safety.RESULTS SS,FAS and PPS was 119,114 and 103 patients in the memantine oral solution group,and 118,115 and 102 in the memantine tablet group,respectively.The ADAS-Cog and ADL total scores at the end of week 4,8, 12,16 decreased significantly compared with baseline for each group(P0.01),and there were no significant differences in ADAS-Cog total scores,ADL total scores and improving level of CGI between the two groups at every time-point(P0.05).The adverse event rate was 20.2%in the memantine oral solution group and 23.7% in the memantine tablet group with no significant difference(P0.05).CONCLUSION Memantine oral solution has good efficacy and safety in treatment of patients with moderate to severe Alzheimer's disease.