During a median follow-up period of 3.9 years , participants who were assigned to extended-release niacin-laropiprant had an LDL cholesterol level that was an average of 10 mg per deciliter ( 0.25 mmol per liter as measured in the central laboratory ) lower and an HDL cholesterol level that was an average of 6 mg per deciliter ( 0.16 mmol per liter ) higher than the levels in those assigned to placebo .