Subject. The scientific works of recent years show an increase in the degree of negative impact of vitamin deficiency and vitamin-like substances on the state of health of the population. With the deficiency of most vitamins, synthetic processes and regeneration of oral tissues are reduced, so often the initial signs of hypovitaminosis are stomatitis, gingivitis, glossitis, and therefore, it is dentists who are the first to diagnose deviations in the body associated with vitamin deficiency. This justifies the relevance and practical value of studying and describing clinical cases of manifestations of deficient vitamin conditions in the oral cavity. The object – is to study the effect of B vitamins on the oral mucosa in order to increase the effectiveness of diagnosis of vitamin-deficient conditions of the human body. Methodology. These clinical examples illustrate the management experience of patients with manifestations of deficient vitamin conditions on the oral mucosa. Clinical and laboratory methods of diagnosing the analysed conditions of the organism were applied. Statistical processing of materials was carried out using the STATISTICA 10 software (StatSoft, Inc., USA). Results. The deficient condition of the organism in relation to vitamins B2, B6, B12 in patients 18-75 years old has been studied. Characteristic clinical changes on the oral mucosa of the examined patients were established. So, in most patients with vitamin B2 deficiency, the classic Sebrel triad was found: dermatitis, glossitis, cheilitis. In those examined with a lack of vitamin B6, language desquamations (smoothed, polished language) were determined in the 83.6%, often combined with glossodinia. Patients with vitamin B12 deficiency were characterized by a lesion in the form of Meller-Gunter glossitis in 74.9% of cases, moreover, 67.6% of patients showed paresthesia in the area of tongue and oral mucosa. Conclusions. The study found that the first clinical symptoms of deficient conditions of the presented vitamins of group B were found from the oral cavity. Thus, it is the dentist who is the first to diagnose the pathological states of lack of group B vitamins in the human body, which emphasizes the importance and relevance of continuing to study these states of the body.