Introduction. In recent years, diarrheal syndrome is the most common clinically significant negative effect of the antibiotic therapy (ABT), which constitutes a first-priority medical and social problem. The prescription of any antibiotic for any duration of treatment may cause a potential risk of developing antibiotic-associated diarrhea (AAD). In that regard, there is a need for the systematization of the main pathogenetic aspects of the deveopment of AAD and the rationale for the use of probiotics to prevent its development and treatment. Aim. To conduct a comparative evaluation of the efficacy, adherence and tolerability of a synbiotic Floriosa containing Bifidobacterium lactis Bl-04, Lactobacillus acidophilus La-14, Lactobacillus rhamnosus Lr-32, inulin, B vitamins, and an eubiotic Bifiform containing Enterococcus faecium, Bifidobacterium longum for the prevention of the development of AAD during and after the use of ABT in inpatients. Materials and methods. A total of 60 patients, which was used for the ABT in the hospital settings, were included in the study: the 1st group (30 patients) received a synbiotic, the 2nd group (30 patients) received an eubiotic. The efficacy of the preventive administration of drugs was assessed by Day 12 and 28 of the therapy. The methods included the patients’ assessment of the efficacy and satisfaction with treatment, an assay of short-chain fatty acids (SCFAs) in feces via gas-liquid chromatography and a Clostridium difficile Toxin A + B rapid test. Results. No cases of AAD were detected in both groups. The study drugs were comparable in terms of their efficacy assessment by the doctor and patients. A trend towards a higher assessment of the therapeutic effect and satisfaction with synbiotic therapy was observed. Changes in the absolute and relative content of SCFAs in the patients’ feces from the baseline level were established. More pronounced positive changes in the quantitative and qualitative composition of acids due to the treatment were identified in the patients who received the synbiotic and had more than 3 risk factors. Conclusions. The synbiotic Floriosa and probiotic Bifiform are effective drugs to prevent AAD. The synbiotic has advantages in terms of the overall assessment of the efficacy of the treatment and patient satisfaction, provides a pronounced protective effect on the intestinal microbiocenosis status during and after the ABT (as evidenced by the changes in SCFAs level in the feces), can be the drug of choice for the prevention of AAD, including AAD associated with C. difficile in individuals with more than 3 risk factors for the development of AAD.