NEVER take 600mg of niacin or even close to that! The Food and Nutrition Board recommends 14mg to 18mg of niacin per day for an adult. A niacin overdose—over 35mg per day—may negatively affect your health in several ways. Excessive amounts of niacin might make your hair fall out. Additionally, rashes, itching and skin flushing may appear. It can damage your liver. Any comments?