Introduction. Generics account for over 80 % of the Russian pharmaceutical market, and their lower price is traditionally regarded as the main mechanism for increasing the economic availability of therapy. In paediatric practice this mechanism is limited by age contraindications and dosage form requirements, which calls for a separate quantitative assessment. Purpose. To carry out a pharmacoeconomic assessment of the availability of drug therapy for gastrointestinal diseases in children with original and generic medicines and to identify the factors limiting the feasibility of the economic substitution potential. Materials and methods. The assessment was performed on a set of real outpatient prescriptions for 2023 (35,632 prescriptions, 5,528 patients, 535 trade names). Cost-minimisation analysis and budget impact analysis were applied; calculations were made for a model patient weighing 20 kg at average retail prices, followed by a sensitivity analysis at the extreme values of the registered price range. Data processing was performed in Python 3.11 (pandas 2.0, numpy 1.24). Results. The economic effect of substitution is determined by the pack size purchased and by the correspondence of the dosage form to the age of the patient: in the ursodeoxycholic acid group, a generic in a comparable pack reduces the course cost by 7.2 %, while in a small pack it increases the cost by 2.8 %, whereas switching to an age-appropriate dosage form reduces the cost by 32.1 %. The theoretical substitution potential in the proton pump inhibitor group (87.2 %) is feasible for only 20.6 % of prescriptions due to age restrictions. Discussion. Annual savings when replacing half of the prescriptions amount to 581,821-730,304 RUB for a single organisation. Three of four results are stable across the whole range of registered prices; in the pancreatin group an unfavourable price combination makes substitution 25.4 % more expensive. Conclusions. Three mechanisms for increasing the economic availability of therapy were identified, differing in the magnitude of the effect, resistance to retail price fluctuations and conditions of applicability; the priority is not the maximisation of price savings but the choice of an option permissible under age restrictions and dosage form requirements.