A young adult ingests a massive quantity of loperamide tablets purchased over the counter in an attempt at self-treatment and develops severe respiratory depression requiring naloxone. In therapeutic doses loperamide never produces such effects. What explains both observations?

• A Megadoses induce CYP3A4, converting loperamide into a central acting metabolite
• B Therapeutic doses are fully inactivated by gastric acid, whereas megadoses overwhelm gastric degradation
• C Loperamide antagonizes mu receptors at low doses and agonizes them at high doses
• D At usual doses, P-glycoprotein actively pumps loperamide out of the blood-brain barrier, but megadoses saturate this efflux pump ✓

Explanation

Loperamide is a potent mu agonist with no intrinsic ability to cross the blood-brain barrier at standard doses because P-glycoprotein efflux transporters exclude it from the CNS, so it acts only on gut opioid receptors. Extremely high doses taken for euphoric or cardiotoxic purposes saturate this efflux system, allowing central respiratory depression. No metabolic activation or dose-dependent receptor switching occurs, which kills options B, C, and A.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.