Vitamin B3 level, a niacin or nicotinic acid test, is an essential nutrient in body metabolism and nervous system function. Measuring the concentration of niacin helps to assess deficiency or excess. Its deficiency can result in pellagra, characterized by symptoms such as diarrhoea, neurological weakness, dermatitis, diarrhoea and dementia. Excessive intake leads to flushing, itching and liver toxicity.