While niacin flush is a relatively harmless side effect, very high doses of niacin can profoundly affect the body. Since the liver processes vitamins, doses of 1,000-3,000 mg of niacin daily ...
Very high doses of 1,000 to 3,000 mg or more of niacin can cause more serious side effects, such as liver damage, decreased blood pressure, dizziness, fatigue, nausea, heartburn, or blurred vision.