"Flag this Response I have been taking vitamins for years, including large doses of vitamin C, so it never occured to me that I might have a problem with niacin.I heard a local radio talk show MD recommend niacin, so I picked up a bottle of 500 mg pills and took my first (and last) after lunch this mid-afternoon.I would already taken a multivitamin with breakfast, but this 500 mg booster didn't sit well" . . . .