Research done outside a living organism — in cell cultures, test tubes, or biochemical assays. Useful for understanding mechanism; insufficient on its own to prove clinical benefit.Latin: "in glass." In vitro data characterizes mechanism, identifies receptor interactions, and generates hypotheses. It cannot account for metabolism, distribution, protein binding, off-target effects in complex tissues, or compensatory physiological responses. On this site, in vitro data alone does not qualify a compound for any clinical evidence tier. "Works in cell culture" is the beginning of a research program, not the end.