An unbalanced diet and some other factors are the reason for the extreme prevalence of hypovitaminosis among the population of both developing and developed countries. The metabolic relationships of B vitamins determine the more frequent occurrence of poly-, rather than monovitamin deficiency. Since B vitamins play an important role in the processes of energy production and in the functioning of neurotransmitter systems, the first symptoms of such polyhypovitaminosis are signs of brain dysfunction: asthenia, depression, anxiety, cognitive disorders. Correct diagnosis of polyhypovitaminosis based on the identification of risk groups of patients and the determination of their subclinical or early manifesting signs of brain dysfunction is the most important task of a practicing physician. Detection of insufficient intake and vitamin content, as well as verification of the onset of symptoms of brain dysfunction and the appointment of adequate therapy can significantly increase the effectiveness of treatment, as well as prevent asthenic, psycho-emotional and cognitive disorders in patients with polyhypovitaminosis.