Ascorbic acid (vitamin C) is a vital nutrient that belongs to the group of antioxidants. Vitamin C plays an important role in the functioning of the central (CNS) and peripheral nervous system (PNS), including maturation and differentiation of neurons, formation of myelin, synthesis of catecholamines, modulation of neurotransmission and antioxidant protection. Neurological diseases and mental disorders are characterized by increased generation of free radicals. At the same time, the highest concentrations of vitamin C are found in the brain and neuroendocrine tissues. It is believed that vitamin C can affect the age of debut and the course of many neurological diseases and mental disorders. However, its potential therapeutic role continues to be studied. The efficacy and safety of vitamin C is likely influenced by the pharmacogenetic profile of the patient, including the carriage of single-nucleotide variants (SNVS), candidate genes associated with vitamin C metabolism in the human body in normal and neuropsychic disorders. The purpose of this thematic review is to update current knowledge about the role of vitamin C pharmacogenetics in the efficacy and safety of its use in neurological diseases (amyotrophic lateral sclerosis, multiple sclerosis, Parkinson's disease, Huntington's disease, Alzheimer's disease, etc.) and mental disorders (depression, anxiety, schizophrenia, etc.). Special attention is paid to the possibility of translating the results of pharmacogenetic studies into real clinical practice in neurology and psychiatry.