Table 4.

Summary of differences between neonatal and adult physiology that affect absorption/distribution of drugs (extracted from [13]).

Route of Administration	Impact on Absorption/Distribution	Reasons
Oral	Altered absorption	Neonatal pH is elevated in the stomach (increased for basic drugs and reduced for acidic drugs) Immature ontogeny of transporter expression
	Reduced absorption	Slower gastric emptying Reduced relative surface area in the intestine
	Increased absorption	Slower intestinal transit Reduced intestinal P-glycoprotein expression
Rectal	Decreased surface area	Reduced relative surface area of rectum
Respiratory	Decreased absorption	Immature lung branching and development Reduced lung capacity and inspiratory flow
Nasal	No data shown	Potential for irritation in the nasal mucosa in neonates
Dermal and transdermal	Increased absorption	Higher BSA/kg ratio Thinner stratum cornea layer More hydrated stratum corneum Higher relative surface area to bodyweight
IV	Reduced distribution	Reduced blood volume
Intramuscular	Reduced distribution	Reduced muscle mass
	Altered distribution	Variable muscle blood flow
Subcutaneous	Reduced distribution	Reduced subcutaneous fat