Gastric acid production 3 mo

Gastric emptying 6-8 mo Hepatic metabolism

Phase I enzyme reactions 5 mo-5 yr

Phase II enzyme reactions 3-6 mo Excretion

Glomerular filtration 3-5 mo

Tubular secretion 6-9 mo

Renal blood flow 5-12 mo

Maternally administered drugs also may affect infants who are breast-fed. Most drugs are present in breast milk in small quantities. However, several drugs can reach concentrations sufficient to adversely affect the newborn. Drugs that are contraindicated during breast-feeding include cocaine, ergotamine, and cimeti-dine. Unfortunately, for many drugs the information regarding risks to the infant from drug in breast milk is not available.

The period from 1 month to 2 years of age is a time of rapid growth and maturation. By the end of this period, most systems function at adult levels. Paradoxically, between 2 and 12 years of age drug clearance greatly increases and often exceeds adult levels. Half-lives are shorter and dosing requirements are frequently greater than for adults (Table 6.3).

From 12 to 18 years of age sex differences start to appear. These differences are often associated with a decreased drug absorption and elimination in the female as opposed to the male. Females have less gastric acidity and an increased gastric emptying time. Estrogens decrease hepatic cytochrome P450 content and therefore may decrease metabolism of some drugs via phase I pathways. Cyclic changes in glomerular filtration are noted during the menstrual cycle.


Oral absorption of drugs is influenced by gastric acidity and emptying time. Gastric acid is rarely found in the

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