It is known that prolonged cell passaging may affect the outcomes of cell-based experiments. This study aimed to comprehensively investigate the consequences of long-term culture of U87MG glioblastoma cells, focusing on cell metabolism, morphology, tumorigenicity and drug response. U87MG (L) cells subjected to long-term culturing (20 passages) displayed altered morphology and significantly enhanced autofluorescence of lipofuscin, a well-established marker of oxidative stress and cellular senescence, compared with control U87MG cells maintained for only 5 passages. Fluorescence-lifetime imaging microscopy (FLIM) of NADH revealed the metabolic shift likely associated with glycolysis in long-term cultured U87MG (L) cells. Using the genetically encoded sensor HyPer7, the increase in basal intracellular levels of hydrogen peroxide in U87MG (L) cells was detected. In vivo studies employing orthotopic intracranial xenotransplantation in immunodeficient NSG mice revealed that, unlike short-term cultured U87MG cells, which formed visible tumor nodules with clearly delineated margins, long-term cultured U87MG (L) cells infiltrated diffusely into brain tissues, invaded bone tissue, and exhibited perineural growth. Such an aggressive behavior resulted in worse survival outcomes of xenograft-bearing mice. Notably, U87MG (L) cells became less susceptible to temozolomide, but acquired sensitivity to death receptor 5 (DR5)-selective variant of cytokine TRAIL in vitro and in vivo due to increased DR5 expression on the cell surface and downregulation of cFLIP expression. Our findings with the U87MG cell line indicate that culture duration can alter cellular responses, thereby impacting experimental outcomes. It should be carefully considered when establishing tumor models and evaluating efficacy of potential drug candidates.