CASP9 and fibrosis: In summary, the present study shows that: i) APAP treatment can induce cell proliferation of kidney fibroblasts even at low doses, and thus we suggest that APAP treatment needs to be carefully monitored in patients with fibrosis; ii) APAP treatment can increase the H2O2 level and activate the caspase-9/-3 cascade to cause cytotoxicity; and iii) the cytotoxic effects of APAP depend on the cell type, with hepatoma cells being more severely affected compared to healthy kidney tubular cells.