Which statement about absorption is true?

• A. Ionized and water-soluble absorbed fastest
• B. Absorption independent of lipid solubility
• C. Lipid soluble and nonionized are absorbed faster
• D. Lipid soluble and ionized are absorbed slower

Answer: (C)

Absorption relies on passive diffusion across membranes, which favors molecules that are lipid-soluble and in the nonionized form. The cell membranes are hydrophobic barriers, so drugs that dissolve in lipids cross more readily. The nonionized (unionized) fraction of a drug is typically more lipid-soluble than the ionized form, so it crosses membranes faster. Therefore, a drug that is both lipid soluble and nonionized is absorbed more quickly than one that is water-soluble or ionized.

In neonates, factors like higher gastric pH can influence the ionization of weak acids and bases, potentially shifting how much is in the unionized form, but the fundamental rule remains: lipid solubility and nonionization drive faster absorption.