Objectives To estimate the incidence, characteristics and outcomes of 10-fold or greater or a tenth or less medication errors in children aged <16 years in Wales.

Design Population-based surveillance study July 2017 to June 2019. Cases were identified by paediatricians and hospital pharmacists using monthly electronic Welsh Paediatric Surveillance Unit (WPSU) reporting system.

Patients ‘Definite’ incident occurred when children received all or any of the incorrect dose of medication. ‘Near miss’ was where the prescribed, prepared or dispensed medication was not administered to the child.

Main outcome measures Incidence, patient characteristics, setting, drug characteristics, outcome, harm and enabling or preventive factors.

Results In total, 50 10-fold errors were reported; 20 definite and 30 near miss cases. This yields a minimum annual incidence of 1 per 3797 admissions, or 4.6/100 000 children. Of these, 43 were overdoses and 7 underdoses. 33 incidents occurred in children <5 years of age. Overall, 37 different medications were involved with the majority, 31 cases, being administered enterally. Of these 31 enteral medication errors, all definite cases (10) had received liquid preparations. Temporary harm occurred in 5/20 (25%) definite cases with one requiring intensive care; all fully recovered.

Conclusions In this first ever population surveillance study in a high-resource healthcare system, 10-fold errors in children were rare, sometimes prevented and uncommonly caused harm. We recommend country-wide improvements be made to reduce iatrogenic harm. Understanding the enabling and preventive factors may help national improvement strategies to reduce these errors.