Why can some basic drugs be shifted back from the bloodstream to the stomach after absorption?

• A. Due to increased gastric motility
• B. Because of pH differences causing ion trapping
• C. Because of hepatic metabolism
• D. Because of renal reabsorption

Answer: (B)

Ion trapping due to pH differences between compartments is what allows some basic drugs to shift back from the bloodstream into the stomach after absorption. A weak base exists in two forms: unionized (lipid-soluble) and ionized (charged). Only the unionized form crosses membranes readily. In the blood (about pH 7.4), the drug is partly unionized and can diffuse. When it encounters the stomach’s very acidic environment, it picks up a proton and becomes BH+, a charged form that cannot cross membranes easily. This ionized form gets effectively trapped in the stomach, making it harder for the drug to return to the blood. Thus, the pH difference creates a one-way movement—ion trapping—that explains why basic drugs can be shifted back toward the stomach after absorption. (This concept contrasts with how acids behave and why factors like gastric motility, hepatic metabolism, or renal reabsorption aren’t the primary drivers of this back-shift.)