The most common adverse reactions that were observed during clinical trials were diarrhea, nausea, abdominal pain, headache, vomiting and rash. Sometimes hypersensitivity reactions and drug fever are observed. After rectal administration, local reactions such as itching, rectal discomfort and urge to defecate are possible. It is possible to develop increased fatigue, paresthesia, methemoglobinemia. The frequency of side effects reported during clinical trials and during post-marketing follow-up is determined as follows: often (from ≥1% to <10%), rarely (from ≥0.01% to <0.1%), very rarely (from <0.01%), the frequency is unknown. From the blood and lymphatic system: very rarely – eosinophilia (as a component of an allergic reaction), anemia, aplastic anemia, leukopenia (including granulocytopenia and neutropenia), thrombocytopenia, agranulocytosis, pancytopenia. From the immune system: very rarely — pancolitis, drug fever, in some cases — hypersensitivity reactions. From the nervous system: often – headache, rarely – vestibular reactions, dizziness, very rarely — peripheral neuropathy, benign intracranial arterial hypertension (in adolescents in puberty). From the heart: rarely – myocarditis * and pericarditis*. From the respiratory system, chest organs and mediastinum: very rarely — allergic and fibrotic changes from the lungs (including shortness of breath, cough, bronchospasm, allergic alveolitis, pulmonary eosinophilia, interstitial lung disease, pulmonary infiltration, pneumonitis). From the gastrointestinal tract: often – diarrhea, abdominal pain, nausea, vomiting; rarely – elevated amylase levels, acute pancreatitis *, flatulence; very rarely – exacerbation of colitis symptoms. From the liver and biliary tract: very rarely — impaired liver function (including increased levels of liver enzymes, cholestasis, bilirubin), hepatotoxicity (including hepatitis*, cholestatic hepatitis, cirrhosis, liver failure). From the skin and subcutaneous tissue: often – rash (including urticaria, erythematous rash), rarely – photosensitization reactions, very rarely – reversible alopecia, Quincke’s edema. From the musculoskeletal system and connective tissue: very rarely — myalgia, arthralgia, reactions similar to lupus erythematosus. From the kidneys and urinary system: very rarely — impaired renal function (including interstitial nephritis* (acute and chronic), nephrotic syndrome, renal failure), urine discoloration, renal failure, which may disappear when the drug is discontinued. From the genital organs: very rarely — oligospermia (reversible). General disorders: in some cases — drug fever. *The mechanism of development of mesalazin-induced myo- and pericarditis, pancreatitis, nephritis and hepatitis is unknown, but its allergic origin is possible. It should be noted that some of these disorders can be explained by intestinal inflammation itself.