Inclusion Criteria:
1. Patient signing the Informed Consent Form and parent/adoptive parent signing the Parent/Adoptive Informed Consent Form.
2. Male and female children between the ages of 6 and 17 years 10 months inclusive at the time of screening.
3. Clinical diagnosis of acute respiratory viral upper respiratory tract infection (J00-J06 according to ICD-10) or acute bronchitis (J20, J21 according to ICD-10).
4. Onset of disease symptoms no more than 3 days prior to screening.
5. Presence of dry non-productive cough in the patient.
6. Frequency of cough attacks ≥10 in the last 24 h before the screening visit and randomization.
7. Cough frequency rated by the patient (for patients 13-17 years of age) or parent/adoptive parent (for patients 6-12 years of age) as 3-4 points on the "daytime cough" section and 2-4 points on the "nighttime cough" Daytime and nighttime cough scales.
8. Patient (for patients 13-17 years of age) or parent/adoptive parent (for patients 6-12 years of age) assessment of cough severity on the digital rating scale as ≥4 points.
9. No indication for therapy due to BHSA infection at the time of inclusion in the study: negative rapid test for group A β-hemolytic streptococcus.
10. For patients 14-17 years of age inclusive, consent to use reliable contraceptive methods throughout study participation and for 3 weeks after the end of the study.
Reliable contraceptive methods are: sexual rest, condom use in combination with spermicide.
Non-inclusion Criteria:
1. Known or suspected hypersensitivity to the active substance or any of the excipients of the study drug/placebo.
2. Known or suspected hypersensitivity to acetylcysteine.
3. Lactose intolerance, lactase deficiency, glucose-galactose malabsorption.
4. Trauma (including open wounds in the oral cavity and erosive desquamous lesions of the oral mucosa) and/or burns of the oropharynx, scarlet fever, rubella, measles, epidemic mumps at the time of screening or within 3 months before screening.
5. Acute obstructive laryngitis or suspected obstructive laryngitis.
6. Presence of signs of laryngeal stenosis (stridor, dyspnea).
7. Exacerbation of chronic bronchitis.
8. Bronchial asthma, including history.
9. Tuberculosis in the anamnesis.
10. Emphysema of the lungs in the anamnesis.
11. Acute or chronic pneumonia or suspected pneumonia.
12. Condition after intubation.
13. Gastroesophageal reflux being the primary cause of cough (in the opinion of the investigating physician).
14. Allergic rhinitis being the primary cause of cough (in the opinion of the physician-researcher).
15. Body temperature \> 39.0 °C.
16. Positive laboratory test result for SARS-CoV-2 antigen using an immunochromatographic assay at the time of screening1.
17. Need for systemic antibiotic therapy and/or other drugs/procedures on the prohibited therapy/procedure list.
18. Use of analgesics or antipyretics within 12 h prior to screening.
19. Use of glucocorticosteroids, β-adrenoblockers, ACE inhibitors, theophylline preparations, expectorants, cough suppressants, anesthetics, anti-allergic drugs (leukotriene receptor antagonists, H1-histamine receptor blockers, sodium cromoglycate preparations), antiviral drugs, immunosuppressants, systemic antibacterial agents within 7 days before screening.
20. A history of smoking.
21. History of renal or hepatic impairment.
22. History of oncologic diseases.
23. Serious cardiovascular disease at the time of screening or within 12 months prior to screening, including: severe arrhythmias requiring treatment with antiarrhythmic drugs, unstable angina pectoris, heart and coronary artery surgery, stroke.
24. Other severe, decompensated or unstable somatic diseases (any disease or condition that threatens the patient's life or worsens the patient's prognosis and makes it impossible for the patient to participate in the clinical trial).
25. Unwillingness or inability of the patient/parent/adoptive parent to comply with the protocol procedures (in the opinion of the investigating physician).
26. Pregnancy or breastfeeding period (for female patients).
27. Alcoholism, drug dependence, substance abuse history and/or at the time of screening.
28. A history of schizophrenia, schizoaffective disorder, bipolar disorder, or other psychiatric pathology.
29. Participation in another clinical trial within 3 months prior to inclusion in the study.
30. Other conditions that, in the opinion of the investigating physician, preclude the patient's inclusion in the study.
Exclusion criteria:
1. Identification of a confirmed case of COVID-19.
2. Ineffectiveness of therapy - persistence or increase in cough attack frequency ≥1 by Visit 3 (Day 8-9) compared to Visit 1 (Day 1).
3. Adverse events (AEs) requiring withdrawal of study drug/placebo.
4. Occurrence of any disease or condition during the study that, in the opinion of the investigator, worsens the patient's prognosis and makes it impossible for the patient to continue participating in the clinical trial.
5. Erroneous inclusion of a patient who does not meet the inclusion criteria and/or meets the criteria for non-inclusion.
6. Taking any of the drugs of prohibited therapy, performing a prohibited procedure.
7. Need to prescribe a prohibited concomitant therapy/procedure.
8. Pregnancy of the patient.
9. Willingness of the patient and/or his/her parent/adoptive parent to terminate the patient's participation in the study.
10. Lack of adequate cooperation of the patient and/or his/her parent/adoptive parent with the physician-researcher during the research process.
11. Other protocol violations that in the opinion of the physician-researcher are significant.
12. Other administrative reasons.
