The success rate in the development of new drugs is generally low and dropout due to safety issues in late phase clinical trials can be a critical blow to a pharmaceutical company. In small molecule drug discovery, not only on-target toxicities but also off-target toxicities (such as genotoxicity and QT prolongation) should be considered early to prevent dropout in later phases, and related guidelines have also been established. Before the first in human dosing phase, in vivo toxicity studies are conducted to determine a safe initial dose for clinical trials, while in vivo toxicity studies at the early drug discovery stage are conducted to identify the toxicity potential of lead compounds. In addition, target safety assessments are also performed to estimate what kinds of on-target toxicity will occur in normal tissues and what kinds of non-clinical safety assessments are needed for further drug discovery. In some cases, knockout animals are used to obtain proof of concept. If critical on-target toxicity is identified, the project might be terminated depending on the indications. However, off-target toxicity may be avoided if the mechanism of toxicity is elucidated by additional experiments. In this presentation, I will introduce our early drug discovery strategy for small molecule drugs and share examples of our approach to elucidate the mechanisms of on-/off-target toxicity.