Why are drug doses for children often based on body weight or body surface area?

Drug dosages for children are calculated based on body weight or body surface area because children are not simply small adults; their physiological processes differ significantly from those of adults.1

During childhood, metabolic rates are much higher, and factors such as bowel motility and gastric acidity can vary, affecting how drugs are absorbed. Furthermore, drug distribution within the body changes throughout development; for example, premature infants may have a higher proportion of body water and less fatty tissue than adults. Because a child's rate of metabolism and excretion changes as they grow, dosage calculations must account for these developmental shifts to ensure safety and efficacy.12

While age-based fractions of adult doses may be used for drugs with a wide margin of safety, this method is often unreliable due to the significant variation in size among children of the same age. Therefore, using weight (milligrams per kilogram) or body surface area (milligrams per square metre) provides a more precise approach to managing pharmacokinetics in a growing body.1