A pharmaceutical composition comprising a tannin selected from 2-O-digalloyl-1,3,4,6-tetra-O-galloyl--D-glucose, 3-O-digalloyl-1,2,4,6-tetra-O-galloyl--D-glucose, 6-O-digalloyl-1,2,3,4-tetra-O-galloyl--D-glucose, 2,6-bis-O-digalloyl-1,3-di-O-galloyl--D-glucose, and 6-O-trigalloyl-1,2,3-tri-O-galloyl--D-glucose. The tannin may be derived from Quercus infectoria. The pharmaceutical composition is used in a method of treating cancer, particularly oral cancer.