OBJECTIVE: To examine if the weight of a child determines adverse events following oral antibiotics prescription.

DESIGN: Population respective cohort using linked general practice (GP), hospital data and linkage with the Welsh Demographic Service for demographic information. Data linkage was performed using Wales health data, extracted from the SAIL (Secure Anonymised Information Linkage) databank.

INCLUSION: Children (0-12 years) prescribed oral antibiotics by their GP in Wales.

EXPOSURE: Antibiotic prescription (penicillins, cephalosporins, macrolides, dihydropyrimidines, nitroimidazoles, nitrofurans, lincosamides).

OUTCOME: Adverse event as defined by; patients' death within 5 days, records of emergency admission within 5 days and GP records of adverse drug reactions or prescription of another antibiotic within 14 days.

ANALYSIS: Logistic regression of adverse events versus no adverse events at follow-up time.

RESULTS: There were 139 571 prescriptions of the selected antibiotics and 71 541 children (51.39% male) included with follow-up data of which there were 25 445 (18.23% of all prescriptions) children experienced adverse outcomes. There was higher odds of adverse events for lower weight children and those who were younger, female, of Asian origin or deprived.

CONCLUSION: The findings support the hypothesis that smaller children for their age (eg, low weight, female, Asian) are more likely to experience adverse events following antibiotics prescription. This work suggests child weight, in addition to age, should be used when prescribing antibiotics to children in primary care.