The pH of the gastrointestinal environment significantly affects drug absorption because it influences the ionization state of the drug. Only the non-ionized form of a drug can readily cross cell membranes by passive diffusion. Since different areas of the GI tract vary in pH, drugs may be absorbed optimally in specific regions. For example, weakly acidic drugs are better absorbed in the acidic stomach, while weak bases absorb better in the more alkaline intestines. This factor directly impacts the bioavailability and therapeutic efficacy of orally administered drugs.

Reference: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, Brunton et al., 13th Edition.