Coenzymes play crucial roles in cellular metabolism and regulation of cellular functions. Among various coenzymes, nicotinamide coenzymes, encompassing coenzyme I (NADH/NAD+) and coenzyme II (NADPH/NADP+), are particularly important due to their biological functions. Maintaining homeostasis of NAD+/NADH and NADP+/NADPH while allowing dynamic changes to happen is crucial for normal physiological activity. Coenzymes extensively participate in metabolic processes related to biosynthesis, energy production, mitochondrial function, calcium homeostasis, regulation of antioxidant/oxidative stress, signal transduction and gene expression, immune activity, aging, and cell death. There is a high correlation between nicotinamide coenzymes and a broad range of human diseases such as intestinal diseases, kidney diseases, cardiovascular diseases, cancer, nervous system diseases, and diabetes. Coenzymes and cellular signal regulatory molecules are involved in the physiological and biochemical processes of cells.