Provided herein are methods for treating schizophrenia or schizoaffective disorder by administering to a subject in need thereof (S)-2-amino-3-methyl-butyric acid (2R,3R,11bR)-3-isobutyl-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-pyrido[2,1-a]isoquinolin-2-yl ester, or an isotopic variant thereof, or a pharmaceutically acceptable salt or polymorph thereof.
