Objective Acetaminophen is the most common medication prescribed in children’s hospitals. The aim of the study was to estimate the frequency and risk factors for acetaminophen underdosing and overdosing in the paediatric intensive care unit (PICU).

Design Retrospective cohort of drug administrations in a large tertiary care PICU.

Patients All PICU admissions, less than 18 years of age, admitted between 1 January 2008 and 1 January 2018, having received at least one dose of enteral acetaminophen.

Methods The primary outcome was acetaminophen underdosing and overdosing, defined as doses exceeding the 10% upper and lower limits of the standard reference range (10–15 mg/kg) and 10% above daily maximum dose (75 mg/kg). A generalised estimating equation regression assessed patient risk factors for single underdosing, single overdosing and cumulative daily overdosing of acetaminophen.

Results Of the 147 485 doses of enteral acetaminophen administered, 7814 (5.3%) were single underdoses (1 in every 19 doses) and 4640 (3.1%) were single overdoses (1 in every 32 doses). There were 6813 cumulative overdose days (1 in every 9 patient-days). Risk factors for both underdosing and overdosing included older age and cardiac admission, whereas risk factors for cumulative overdosing were young age and cardiac admission. Electronic prescribing increased the risk of underdosing and overdosing, but decreased cumulative acetaminophen overdosing (relative risk 0.51, p=0.001).

Conclusion Acetaminophen underdosing and overdosing are common in the PICU and can be detected with pharmacoepidemiology. Electronic prescribing increased the risk of single underdosing and overdosing, although it reduced the risk of cumulative overdosing.