Induction of P-glycoprotein will shift the concentration-time curve in which direction?

• A. Upward shift
• B. Downward shift
• C. No change
• D. Undetermined shift

Answer: (B)

P-glycoprotein acts as an intestinal efflux transporter, pumping drugs back into the lumen and reducing their absorption. When P-glycoprotein is induced, more transporter protein is present, so more drug is expelled during absorption and a smaller amount reaches the systemic circulation. This lowers plasma concentrations at all time points, yielding a downward shift of the concentration-time curve. You’ll typically see a lower Cmax and often a reduced AUC, reflecting diminished overall exposure. In contrast, increasing absorption or blocking efflux would cause an upward or unchanged curve, respectively, which is not what induction does.