in the first variant , the pharmaceutical composition comprises antibiotic and prebiotic in the form of oligosaccharide of a group comprising fructooligosaccharides , galactooligosaccharides , maltooligosaccharides and isomaltooligosaccharides the polymerisation degree of which ranges from 2 to 10 , the particle size is equal to less than 0 . 3 mm and the purity is of at least 95 %, wherein the antibiotic particle size ranges from 20 to 200 mkm and the antibiotic and oligosaccharide are contained with a mass ratio ranging from 1 : 1 to 1 : 100 respectively . in the second variant , the pharmaceutical composition comprises antibiotic which is in the form of a powder with the particle size ranging from 20 t0 200 mkm and is selected form a group consisting of beta - lactams , including the combination thereof with bacterial betalactamase inhibitors , azalides , fluoroquinalones , amphenicols , glycopeptides , ansamycins , nitrofurans , phosphonic acid derivatives , cycloserine and trimetoprim . the prebiotic is in the form of a powder oligosaccharide , the polymerisation degree of which ranges from 2 to 10 , the particle size is equal to less than 0 . 3 mm and the purity is of at least 95 %, wherein the antibiotic and oligosaccharide are contained with a mass ratio ranging from 1 : 1 to 1 : 100 respectively . the above - mentioned compositions are used for preventing intestinal dysbiosis during antibiotic therapy and are perorally administered . the inventive composition produces a stimulating effect on intestinal lactobacilli and bifidobacteria simultaneously inhibiting the growth and proliferation of extraneous or pathogenic microflora .