Severe deficiency of Vitamin B3 will lead to a condition known as pellagra.
Symptoms of pellagra will include a scaly rash, swollen mouth, vomiting and diarrhoea, disorientation and memory loss - amongst others.
This is a severe condition and can be life threatening. Lesser or short term deficiency may cause indigestion, fatigue, low mood and sores.
B3 deficiency is very rare in the western world as there are plentiful dietary sources of the vitamin. The most common cause is high alcohol intake, which can reduce the amount absorbed from the diet.
Due to the large variability in the metabolism of nicotinic acid, the dosing preparation used (immediate-release vs. extended release), and the mg doses used, the serum concentrations may range from <20 ng/mL to about 30,000 ng/mL.