Which statement best describes how GI pH affects drug absorption?

• A. pH influences solubility and dissolution, and thus absorption.
• B. pH has no effect on drug solubility.
• C. Permeability is unaffected by ionization state.
• D. Absorption rate is only dependent on dose, not pH.

Answer: (A)

GI pH shapes how a drug exists in solution, i.e., its ionization state. The ratio of ionized to unionized forms depends on the drug’s pKa and the local pH, and this ratio governs both solubility and permeability. Ionized forms tend to be more soluble in water, which helps with dissolution, but they cross membranes poorly. Unionized forms permeate membranes more readily, increasing absorption, but they often solubilize less well. Because dissolution (solubility-driven) is typically the rate-limiting step for many oral drugs, changes in pH along the GI tract can speed up or slow absorption by altering how much drug dissolves versus how well it can cross the intestinal lining. That’s why pH affects absorption through its impact on solubility and dissolution. The other statements ignore these pH-dependent effects on solubility and permeability, so they don’t fit.