Chemical structures of the indole alkaloids harmaline, ibogaine, and ibogaline. Harmaline, ibogaine, and ibogaline share an indole nucleus and have nearly identical types of behavioral effects. Zetler et al. (1972, 1974) reported that the positions and number of methoxy groups (left) greatly influence tremorigenic potency, whereas the additional ring structures (right) have little effect on drug action within this group of compounds. For comparative potencies of tremor induction, seeZetler et al. (1972, 1974).