Vitamin B3 refers to the molecule commonly called nicotinic acid, or niacin, though it may also refer to the other vitamin B3 vitamer, called nicotinamide. Vitamin B3 is necessary to support the function of many enzymes.
Niacin supplementation is very effective at normalizing blood lipid levels. People with low HDL-C levels supplementation experience an increase in HDL-C levels, while people with high LDL-C experience a reduction in LDL-C levels. Triglyceride levels also fall after supplementation, which makes niacin look like a great cardioprotective supplement on paper. Unfortunately, niacin supplementation does not result in reduced cardiovascular disease risk, since it also increases insulin resistance, which negates the benefits niacin provides for blood lipid levels.
Other benefits of niacin supplementation are theorized to extend to growth, cognition, and longevity. This is because niacin supplementation increases cellular nicotinamide adenine dinucleotide (NAD+) levels. Preliminary evidence suggests increased NAD+ levels may result in the above benefits, but much more research is needed to determine if this effect actually occurs. Topical application of nicotinamide is sometimes used for skin health, though it is not as effective as vitamin A. Nicotinamide is used for topical application because it does not result in the flushed skin that niacin supplementation can cause.
Current evidence suggests prolonged niacin supplementation increases insulin resistance because it hinders the ability of insulin to suppress glucose synthesis in the liver. This causes an increase in blood glucose levels, which leads to lowered insulin sensitivity over time, since the relevant receptor is eventually desensitized to the elevated glucose levels in the blood. The flush caused by niacin supplementation is a temporary effect. Though it may be uncomfortable, it is not harmful. There are many case studies describing people overdosing on niacin in an effort to pass a urine test. Niacin overdose results in multiple organ failure and is not effective at masking a urine test.