Nicotinamide N-methyltransferase (NNMT) is an enzyme involved in nicotinamide metabolism, primarily expressed in the liver but also found in other tissues such as the kidneys, lungs, and adipose tissue. NNMT transfers a methyl group from S-adenosylmethionine to nicotinamide, producing N-methylnicotinamide and S-adenosylhomocysteine, participating in various biological reactions and signaling pathways, such as energy metabolism, DNA methylation, and inflammatory responses. NNMT gene expression is tissue-specific, and its levels and activity vary in diseases such as cancer, obesity, and diabetes. Mutations and abnormal expression are associated with the onset and progression of these diseases.