The first version of SMPDB was released on January 1, 2010. This release contained more than 350 image-mapped pathways for small molecule pathways.  The viewer interface was limited to scroll-bar image navigation with 3-step (small, medium, large) zooming. The pathways in this first version were limited to 1) human metabolic pathways; 2) human metabolic disease pathways; and 3) human metabolite signaling pathways.  The second version of SMPDB was released in 2014. This version contained more than 620 small molecule pathways.  The viewer interface was enhanced to include a Google-Map-like interface with click-n-drag image navigation and unlimited, interactive zooming. The pathways in this second version were expanded to include: 1) general human metabolic pathways; 2) human metabolic disease pathways; 3) human metabolite signaling pathways; 4) drug action pathways and 5) drug metabolism pathways.