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Background and Importance Accurate reconstitution of oral powder medications is essential to ensure the correct final drug concentration, particularly in paediatrics and for narrow therapeutic index (NTI) drugs. Discrepancies between theoretical and actual volumes may compromise treatment efficacy and safety. Despite its clinical importance, this aspect is rarely systematically evaluated in routine practice.Aim and Objectives To evaluate the concordance between final volumes obtained after reconstitution of oral powder medications and the theoretical volumes indicated in the package leaflet and Summary of Product Characteristics (SmPC).Material and Methods The hospital formulary was reviewed to identify oral powder or granule formulations requiring reconstitution prior to administration.Selection criteria included: preparations that must be compounded in the hospital pharmacy; drugs commonly used in paediatrics; and/or NTI drugs.Each suspension or solution was reconstituted in triplicate, strictly following the instructions provided in the package leaflet and SmPC. The final volume was measured using graduated cylinders (100 ± 1 mL and 250 ± 1 mL). The measured volume was compared with the theoretical value, and the actual drug concentration and percentage error were calculated. An error margin of ±10% was considered acceptable according to the European Pharmacopoeia.Results Seven out of 13 formulations were selected: Amoxicillin 250 mg/5 mL, Amoxicillin–clavulanic acid 100 mg/mL–12.5 mg/mL, Azithromycin 40 mg/mL, Erythromycin 100 mg/mL, Linezolid 100 mg/5 mL, Valganciclovir 50 mg/mL, and Mycophenolate mofetil 1g/5 mL.Five of the 7 (71.4%) showed no significant deviation (error ≤ ±10%). Erythromycin and linezolid exceeded this margin, showing a mean final volume of 120.17±0.76 mL (83.22 mg/mL; mean error 20.17±0.76%) and 169.33±0.76 mL (17.72 mg/mL; mean error 12.89±0.51%), respectively.Conclusion and Relevance The discrepancies observed highlight the importance of verifying the final volume and concentration after reconstitution, especially for paediatric and narrow therapeutic index drugs. Errors greater than ±10% may represent a significant quality deviation, potentially compromising treatment efficacy and safety. Increasing the number of oral powder formulations analysed would help confirm the clinical significance of these findings and support the need to update leaflet and SmPC information regarding actual final volume and concentration.Conflict of Interest No conflict of interest