Abstract Pegylated interferon (peg-IFN) and ribavirin (RBV) are effective in eradicating the hepatitis C virus in more than half of patients. However, anemia arising from RBV-induced hemolysis can prompt dose reductions and lower sustained virologic response (SVR) rates. In early clinical trials, Viramidine (VRD, renamed taribavirin), an RBV prodrug, was associated with less anemia and VRD given at 600 mg twice daily (BID) appeared to provide the best safety with comparable efficacy to RBV. The phase III Viramidine's Safety and Efficacy versus Ribavirin 1 (ViSER1) study randomized 972 treatment-naïve patients with chronic hepatitis C to fixed-dose VRD (600 mg BID) or weight-based RBV (1000 or 1200 mg/day), each given with peg-IFN alfa-2b at 1.5 μg/kg/week. The primary efficacy endpoint was SVR rate, and the primary safety endpoint was hemoglobin (Hb) event rate (percent of patients with Hb < 10 g/dL or at least a 2.5-g/dL decrease from baseline). SVR rates were 37.7% with VRD (244/647) and 52.3% with RBV (170/325). Thus, the ViSER1 study failed to demonstrate the primary noninferiority efficacy endpoint. Significantly fewer patients had Hb events with VRD (353/647; 54.6%) compared to those with RBV (272/325; 83.7%) (P < 0.001), and significantly fewer developed anemia (Hb < 10 g/dL) with VRD (34/647; 5.3%) compared to those with RBV (76/325; 23.5%) (P < 0.001). Conclusion: Fixed doses of VRD failed to demonstrate noninferiority to RBV in producing SVR rates. The incidence of anemia was approximately four-fold significantly lower with VRD than with RBV. These results suggest fixed-dose VRD given 600 mg BID is insufficient to treat patients with chronic hepatitis C; a weight-based dosing trial of viramidine is currently under way. (Hepatology 2009.)