Salicylic acid in a patient with gastric achlorhydria is given a standard dose. Which is the predicted serum concentration relative to normal?

• A. Less than expected
• B. Greater than expected
• C. No change
• D. Absorbed completely

Answer: (A)

The amount of salicylic acid that shows up in the blood after a standard dose depends on how much of the drug is in the non‑ionized form, which is the form that crosses membranes readily. Salicylic acid is a weak acid with a pKa around 3, so in the highly acidic stomach most of it is non‑ionized and absorbed well. If gastric pH is raised (gastric achlorhydria), the equilibrium shifts toward the ionized form, which does not cross membranes as easily. That reduces absorption and lowers the resulting serum concentration compared with normal gastric acidity. So the predicted serum concentration is less than expected.