The efficacy and safety of 2 regimens of a combination of a proprietary niacin extended release plus simvastatin (NER/S; 1,000/20 and 2,000/20 mg/day) were compared with simvastatin monotherapy (20 mg/day) for 24 weeks in 319 high-risk patients with predominantly mixed dyslipidemia who were already at National Cholesterol Education Program Adult Treatment Panel III risk-adjusted goals for low-density lipoprotein cholesterol. After a run-in on simvastatin 20 mg/day, both NER/S doses (1,000/20 and 2,000/20 mg/day) resulted in greater decreases in non-high-density lipoprotein (HDL) cholesterol vs simvastatin 20 mg/day (-13.9% and -22.5% vs -7.4%, respectively; p <0.01). Significant improvements in HDL cholesterol, triglycerides, apolipoprotein B, lipoprotein(a), and total/HDL cholesterol ratio were also observed. Patients with hypertriglyceridemia (triglycerides > or =200 mg/dl) typically had greater lipid responses to NER/S with the notable exception that HDL cholesterol responses to NER/S were similar in those with or without increased triglycerides. Treatment with both doses of NER/S was well tolerated; < or =60% of patients in any treatment group experienced flushing, >90% of flushing was mild or moderate in intensity, and only 7.5% of patients in both NER/S treatment groups discontinued because of flushing. The safety of NER/S was consistent with the safety profile of each individual component. In conclusion, this study showed that NER/S provided additional clinically relevant improvements in multiple lipid parameters and was safe and well tolerated.