When you don't have enough niacin in your body, you develop a deficiency known as pellagra. Symptoms of this deficiency include flushed skin and rashes, digestive disorders, and mental impairment. However, large doses of niacin can be unhealthy as well, causing peptic ulcers and skin discomfort. Therefore, this is definitely a vitamin supplement that you should only take under complete medical supervision.
The U.S. Department of Agriculture recommends that adults (over the age of 14) get about 14 to 16 milligrams of niacin every day to stay healthy. This number increases if you are pregnant or breastfeeding and decreases if you are younger or have special dietary needs. Again, the best way to get enough niacin in your body is through food sources, but vitamin supplements can help you receive enough of this vitamin if your are worried you may not be getting enough. 
How can you make sure you have enough niacin in the body? Simply put, it is important to consume foods rich in niacin. If you look on the label, you should see niacin, niacin acid or B3. This is found in milk, cheese, and other dairy products, as well as meats like poultry, fish, and lean beef and nuts. Eggs can give you a good source of niacin as well, and if you eat enough legumes, like beans, you will be sure to be getting enough niacin in your diet. Other products that may contain niacin include enriched foods like breads and cereals. Be sure to check the label. 
Niacin is important in the body for growth, reproduction, and the immune system. It can help you convert food to energy by assisting in the function of the stomach, intestines, and other parts of the digestive system. It is also important for building and regeneration of nerves and skin. If you do not have enough niacin in your body, these parts of your health will be greatly affected.