Curcumin enhances 5-FU to inhibit cell viability of HCT116 and HCT116R cells. HCT116 and HCT116R cells (1×106/ml) were treated with different concentrations of curcumin (0, 0.1, 1, 5, 10, 20 μM) (A), 5-FU (0, 0.01, 0.1, 1, 10 nM) (B) or HCT116 and HCT116R cells were co-treated with curcumin (5 μM) and with 5-FU in different concentrations (0, 0.01, 0.1, 1, 10 nM) (C) in serum-starved medium for 14 days and cell viability was measured using the MTT assay, as described under Material and Methods. Concentrations of curcumin or/and 5-FU resulting in 50% growth inhibition were indicated as individual IC50 values. The results are provided as mean values with standard deviations from at least three independent experiments. OD value at 100% viable cells was for HCT116 (4.4) and for HCT116R (6.7). Values were compared with the control and statistically significant values with p < 0.05 were designated by an asterisk (*) and p < 0.01 were designated by two asterisks (**).