Niacin occurs naturally in many foods, including greens, meat, poultry, fish, and eggs, although in a fraction of the dose shown to achieve changes in cholesterol. Many products are also fortified with niacin during manufacturing. What are the risks of taking niacin? Side effects. Niacin can cause flushing, especially when you first begin taking it. Your health care provider will probably suggest increasing the dose slowly to reduce this problem.