Nicotinic acid

Nicotinic acid, or niacin, is an organic compound and a vitamer of vitamin B₃, an essential human nutrient. It is produced by plants and animals from the amino acid tryptophan.

Nicotinic acid is also a prescription medication. Amounts far in excess of the recommended dietary intake for vitamin functions will lower blood triglycerides and low density lipoprotein cholesterol (LDL-C), and raise blood high density lipoprotein cholesterol (HDL-C, often referred to as "good" cholesterol). There are two forms: immediate-release and sustained-release nicotinic acid. Initial prescription amounts are 500 mg/day, increased over time until a therapeutic effect is achieved. Immediate-release doses can be as high as 3,000 mg/day; sustained-release as high as 2,000 mg/day. Despite the proven lipid changes, nicotinic acid has not been found useful for decreasing the risk of cardiovascular disease in those already prescribed a statin drug. A 2010 review had concluded that nicotinic acid was effective as a mono-therapy, but a 2017 review incorporating twice as many trials concluded that prescription nicotinic acid, while affecting lipid levels, did not reduce all-cause mortality, cardiovascular mortality, myocardial infarctions, nor fatal or non-fatal strokes. Prescription nicotinic acid was shown to cause hepatotoxicity and increase risk of type 2 diabetes. Nicotinic acid prescriptions in the United States had peaked in 2009 at 9.4 million, declining to 800 thousand by 2020. In 2023, it was the 288th most commonly prescribed medication in the US, with more than 500,000 prescriptions.

Nicotinic acid has the formula C
₆H
₅NO
₂ and belongs to the group of the pyridinecarboxylic acids. As the precursor for nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate, it is involved in DNA repair.

Extra-terrestrial nicotinic acid has been found in carbonaceous chondrite meteorites and in sample-returns from the asteroids 162173 Ryugu and 101955 Bennu.

The term "niacin" was originally coined from "nicotinic acid vitamin", with the goal of distancing the nutrient B3 from the drug nicotine in tobacco. As a result, it originally referred to the nutritional entity of vitamin B₃. However, in American English, the term has also come to mean "nicotinic acid" in the context of high-dose use as a prescription medicine. In other varieties of English, the medicine is only ever called "nicotinic acid". The term "nicotinic acid" unambiguously refers to the substance and the prescription medicine containing it, which treats elevated cholesterol and triglycerides. When used as a drug, daily doses range from 500 to 3,000 mg/day. High-dose nicotinamide does not have this medicinal effect.

Vitamin B₃ has several vitamers that can act in place of each other, including nicotinic acid and nicotinamide. It is precursor of the coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). These compounds are coenzymes for many dehydrogenases, participating in many hydrogen transfer processes. NAD is important in catabolism of fat, carbohydrate, protein, and alcohol, as well as cell signaling and DNA repair, and NADP mostly in anabolism reactions such as fatty acid and cholesterol synthesis. Vitamin intake recommendations made by several countries are that intakes of 14–18 mg/day are sufficient to meet the needs of healthy adults. Nicotinic acid and nicotinamide are both used for prevention and treatment of pellagra, a disease caused by lack of the vitamin.

In the United States, nicotinic acid is sold as a non-prescription dietary supplement with a range of 100 to 1000 mg per serving. These products often have a Structure/Function health claim allowed by the US Food & Drug Administration (FDA). An example would be "Supports a healthy blood lipid profile." The American Heart Association strongly advises against the substitution of dietary supplement nicotinic acid for prescription nicotinic acid because of potentially serious side effects, which means that nicotinic acid should only be used under the supervision of a health care professional, and because manufacture of dietary supplement nicotinic acid is not as well-regulated by the FDA as prescription nicotinic acid. More than 30 mg nicotinic acid consumed as a dietary supplement can cause skin flushing. Face, arms and chest skin turns a reddish color because of vasodilation of small subcutaneous blood vessels, accompanied by sensations of heat, tingling and itching. These signs and symptoms are typically transient, lasting minutes to hours; they are considered unpleasant rather than toxic.

Prescription nicotinic acid, commonly labeled as niacin in the United States, is available in immediate-release and slow-release formulations. It 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 800 thousand by 2020. In 2023, it was the 288th most commonly prescribed medication in the United States, with more than 500,000 prescriptions.