These signs and symptoms are typically transient, lasting minutes to hours; they are considered unpleasant rather than toxic. As lipid-modifying medication
In the United States, prescription niacin, in immediate-release and slow-release forms, is used to treat primary hyperlipidemia and hypertriglyceridemia. It is used either as a monotherapy or in combination with other lipid-modifying drugs. Dosages start at 500 mg/day and are often gradually increased to as high as 3000 mg/day for immediate release or 2000 mg/day for slow release (also referred to as sustained release) to achieve the targeted lipid changes (lower LDL-C and triglycerides, and higher HDL-C). Prescriptions in the US peaked in 2009, at 9.4 million and had declined to 1.3 million by 2017. In late 2017, Avondale, having acquired the rights to Niacor from Upsher Smith, raised the price of the drug by more than 800%.Systematic reviews found no effect of prescription niacin on all-cause mortality, cardiovascular mortality, myocardial infarctions, nor fatal or non-fatal strokes despite raising HDL cholesterol. Reported side effects include an increased risk of new-onset type 2 diabetes. Mechanisms
Niacin reduces synthesis of low-density lipoprotein cholesterol (LDL-C), very low-density lipoprotein cholesterol (VLDL-C), lipoprotein(a) and triglycerides, and increases high-density lipoprotein cholesterol (HDL-C). The lipid-therapeutic effects of niacin are partly mediated through the activation of G protein-coupled receptors, including hydroxycarboxylic acid receptor 2 (HCA2)and hydroxycarboxylic acid receptor 3 (HCA3), which are highly expressed in body fat.
This is because the amino acid is utilized to synthesize the vitamin.For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For niacin labeling purposes 100% of the Daily Value is 16 mg. Prior to 27 May 2016 it was 20 mg, revised to bring it into agreement with the RDA. Compliance with the updated labeling regulations was required by 1 January 2020 for manufacturers with US$10 million or more in annual food sales, and by 1 January 2021 for manufacturers with lower volume food sales. A table of the old and new adult daily values is provided at Reference Daily Intake. Sources
Niacin is found in a variety of whole and processed foods, including fortified packaged foods, meat from various animal sources, seafoods, and spices. In general, animal-sourced foods provide about 5–10 mg niacin per serving, although dairy foods and eggs have little. Some plant-sourced foods such as nuts, legumes and grains provide about 2–5 mg niacin per serving, although in some grain products this naturally present niacin is largely bound to polysaccharides and glycopeptides, making it only about 30% bioavailable. Fortified food ingredients such as wheat flour have niacin added, which is bioavailable. Among whole food sources with the highest niacin content per 100 grams:

Vegetarian and vegan diets can provide adequate amounts if products such as nutritional yeast, peanuts, peanut butter, tahini, brown rice, mushrooms, avocado and sunflower seeds are included.