Niacin, also known as Vitamin B3 or nicotinic acid. First synthesized in 1867 when German scientists oxidized nicotine, niacin is found in meat, poultry, red fish and, in smaller amounts, nuts, legumes, and seeds. Some symptoms of deficiency include anemia, headaches, and fatigue. 
Niacin is a precursor to NAD ( nicotinamide adenine dinucleotide) and NADP (nicotinamide adenine dinucleotide phosphate). Niacin also plays a role in forming NADH. Both NAD and NADP are major players in the ETC. NAD+ loses and gains the H+ electron throughout the citric acid cycle and during the electron transport chain.
The Dynamics and Considerations of Niacin Consumption: A Comprehensive Overview
Niacin is generally taken as nicotinic acid or nicotinamide; it is water soluble. This is beneficial because you are less likely to consume too much niacin. However, it is still possible do so. Niacin should only be taken under the supervision of a licensed physician Some signs of niacin toxicity are dizziness, low blood pressure, fatigue, blurred vision, and impaired glucose tolerance. 
Niacin is commonly found in corn, but niacin is only absorbed when it is treated with calcium hydroxide. 
References and Suggested Reading
Brown, B. Greg, et al. “Simvastatin and niacin, antioxidant vitamins, or the combination for the prevention of coronary disease.” New England Journal of Medicine 345.22 (2001): 1583-1592.
Ganji, Shobha H., Vaijinath S. Kamanna, and Moti L. Kashyap. “Niacin and cholesterol: role in cardiovascular disease.” The Journal of nutritional biochemistry 14.6 (2003): 298-305.
Fukushima, Tetsuhito. “Niacin metabolism and Parkinson’s disease.” Environmental health and preventive medicine 10.1 (2005): 3-8.
Gasperi, Valeria, et al. “Niacin in the central nervous system: an update of biological aspects and clinical applications.” International journal of molecular sciences 20.4 (2019): 974.
Kashyap, Moti L., et al. “Niacin for treatment of nonalcoholic fatty liver disease (NAFLD): novel use for an old drug?.” Journal of clinical lipidology 13.6 (2019): 873-879.
Ilkhani, Faranak, Banafshe Hosseini, and Ahmad Saedisomeolia. “Niacin and oxidative stress: a mini-review.” Journal of Nutritional Medicine and Diet Care 2.1 (2016): 014.
Pirinen, Eija, et al. “Niacin cures systemic NAD+ deficiency and improves muscle performance in adult-onset mitochondrial myopathy.” Cell Metabolism 31.6 (2020): 1078-1090.