Currently, lactulose is known to most as a laxative that has a hyperosmotic effect, stimulating intestinal peristalsis. The drug has long established itself as a safe and effective medicine. Lactulose is one of the few drugs that is approved for use in pregnant women and children under 6 months of age with functional constipation. The prebiotic properties of lactulose were discovered in 1957. After research, it was found that it promotes the growth of beneficial bacteria, such as bifidobacteria and lactobacilli. In addition to being used as an effective weak and prebiotic agent, lactulose has been successfully used since 1966 for the treatment of hepatic encephalopathy. The mechanism of action of the drug is that it prevents the absorption of excess ammonia, which is formed in the large intestine, through the hydrolysis of protein and urea by the intestinal microflora. Lactulose, which has a wide range of effects on nitrogen metabolism by the intestinal microflora, affects not only ammonia, but also other bacterial toxins as a result of the studies that have demonstrated the reliable effectiveness of the drug, the world’s leading professional communities have included lactulose in their recommendations as the drug of choice for the treatment of patients with hepatic encephalopathy at any stage of the disease. The article presents current data on the effectiveness and safety of the use of lactulose in various diseases. In addition, attention is paid to such a concept as microbiota. Its functions and influence on the human body are described.