The authors carried out a systematic review and subsequent meta-analysis of randomized clinical trials evaluating the efficacy of the immunomodulator agent cycloferon as tablets in adults and children with viral respiratory diseases. A total estimate of its clinical efficacy was obtained in terms of compared heterogeneous groups and response variables. The data published in 16 articles were used to calculate the formal parameters of the clinical efficacy of cycloferon (increased absolute and relative benefits, odds ratio (OR); the number of patients needed to be additionally treated with cycloferon to achieve a favorable outcome or to prevent a poor outcome in one patient, etc.). High heterogeneity hampered the unequivocal interpretation of results; however, combining the compared homogeneous groups in the meta-analysis (with adjustments for fixed and random effects) increased the statistical power of the investigation. In children aged 6 to 18 years, the OR for the positive effect of the drug (no new cases after its preventive administration) was 5.3 (95% confidence interval (CI), 4.8—5.9), heterogeneity test, χ2 = 249.5; p=0.000...; I2 = 94.8% (95% CI, 92.7—96.3%). This suggested the heterogeneity of clinical trial data and extrapolated this estimate to medical practice. The use of cycloferon in adults to treat acute respiratory viral infection enhanced their chances of enduring the disease in a mild form and avoiding serious complications: the OR for positive outcomes was 9.7 (95% CI, 7.0—13.0), while the effect was more homogeneous than in children (heterogeneity test, χ2 = 7.4; p=0.061...; I2 = 59.4% (95% CI, 0—86.5). Thus, the use of cycloferon to treat and prevent acute viral respiratory infections showed a more than 5-fold increase in the probability of avoiding the disease or enduring the latter in a mild form.