This document discusses regulatory perspectives on metabolite identification and safety testing. It notes that metabolite toxicity profiles may differ from parent drugs, so safety testing is necessary. Metabolites of concern are those that are unique to or disproportionately present in humans. Safety testing should be conducted early in development to avoid delays. The guidance recommends identifying differences between species' metabolism early. If safety testing is warranted, studies should be completed before large clinical trials. The document reviews safety evaluation approaches and timelines to consider for metabolite testing throughout the drug development process.