When One Size Doesn’t Fit All: Aciclovir Dosing and the Obesity Challenge

The global rise in childhood obesity poses major challenges for safe prescribing. Conventional weight-based or body surface area (BSA)- based dosing guidelines often fail to account for differences in body composition, potentially leading to underdosing or toxicity at the extremes of weight centiles. Pharmacovigilance data on aciclovir from the MHRA include >100 paediatric adverse reaction reports since 2015, highlighting an ongoing burden of treatment-related harm1. The British National Formulary for Children (BNFC) recommends weight-based aciclovir dosing up to 3 months and between 12 and 17 years, whilst BSA-based dosing is used for those aged 3 months to 11 years2. For weight-based dosing, the Neonatal and Paediatric Pharmacy Group advises the use of ideal body weight (IBW) for children >98th percentile weight-for-age and sex. Although this approach may lead to clinically implausible dose reductions at arbitrary thresholds3, it serves to moderate exponential dosing patterns. Alarmingly, no such adjustment is proposed for BSA-based calculations, whereby dosing is derived directly from total body weight (TBW). Aciclovir’s volume of distribution correlates more closely with IBW and total body water than TBW, meaning that dosing based on TBW in children with obesity could exceed pharmacokinetically appropriate levels. In addition, the abrupt transition from BSA-based to weight-based calculations at age 12 results in a sharp dosing drop.