If a basic drug, after absorption, shifts back into the stomach due to pH differences and becomes ion trapped there, how would the plasma concentration-time curve be affected?

• A. Curve shifts upward (higher concentrations)
• B. Curve shifts downward (lower concentrations)
• C. Curve shows no change
• D. Curve becomes steeper but remains the same max

Answer: (B)

Ion trapping of a basic drug in the stomach reduces how much active drug reaches the systemic circulation. In the stomach’s acidic environment, a basic molecule becomes protonated and ionized, making it less able to cross membranes. If some drug that has been absorbed into the blood can diffuse back into the stomach, it gets trapped there and is not readily reabsorbed into plasma. That means less drug is available in the bloodstream at any given time, so plasma concentrations are lower throughout the time course. The result is a downward shift in the plasma concentration–time curve, with a lower Cmax and reduced overall exposure.