Nicotinamide-d4 is intended for use as an internal standard for the quantification of nicotinamide by GC- or LC-MS. Nicotinamide is an amide form of niacin, which is also known as vitamin B3, that can be biosynthesized in vivo or obtained through the diet. It is a precursor in the synthesis of the metabolic cofactor NAD+ and an inhibitor of sirtuin 1 (SIRT1; IC50 = <50 µM). Nicotinamide (10 µM) increases the activity of serine palmitoyltransferase (SPT) and the biosynthesis of ceramide, glucosylceramide, sphingomyelin, free fatty acids, and cholesterol in primary human keratinocytes. Nicotinamide (40 µM) induces apoptosis in SNU-398, SNU-739, and HepG2 hepatocellular carcinoma (HCC) cells, and it prevents the formation of neoplastic lesions in a diethylnitrosamine-induced mouse model of HCC. Unlike niacin, nicotinamide does not reduce plasma lipid levels or induce flushing.