Loperamide is an opioid widely used for diarrhea because it does not produce central nervous system effects. The mechanism by which it is restricted from crossing the blood-brain barrier is:
- A It is actively pumped out of the CNS by P-glycoprotein efflux transporters ✓
- B It is highly ionized at physiological pH and cannot cross lipid membranes
- C It is rapidly metabolized in the blood before reaching the brain
- D It cannot cross the blood-brain barrier because of its large molecular size
Correct answer: A. It is actively pumped out of the CNS by P-glycoprotein efflux transporters
Explanation
Loperamide is a substrate for P-glycoprotein, an efflux transporter at the blood-brain barrier that actively pumps it back into the bloodstream, preventing CNS penetration and thus avoiding central opioid effects. This is a key pharmacological distinction from other opioids. High doses combined with P-gp inhibitors can overcome this mechanism and produce CNS effects.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.