TOXICOLOGY
When Safe Becomes Deadly: Loperamide Toxicity in the Era of Opioid Misuse SAEM PULSE | NOVEMBER-DECEMBER 2025
By Liz Uttaro, PharmD and Renee Petzel Gimbar, PharmD
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Background
Loperamide is a phenylpiperidine opioid commonly used as an overthe-counter antidiarrheal medication. It stimulates mu-opioid receptors in the gut and blocks intestinal calcium channels, slowing intestinal transit time. It also has antisecretory properties that reduce diarrhea. When taken at higher doses, loperamide can penetrate the central nervous system (CNS) and cause serious adverse effects including respiratory depression,
CNS depression, and cardiac dysrhythmias, which can be fatal. After oral ingestion, loperamide undergoes extensive first-pass metabolism, resulting in extremely poor bioavailability and limited systemic circulation. Its metabolism primarily involves the CYP3A4 and CYP2C8 enzymes, which convert it to inactive metabolites. Additionally, P-glycoprotein (P-gp) efflux pumps (Figure 1) push loperamide back into the intestinal lumen and bile, preventing accumulation in the CNS and gastrointestinal tract.
Figure 1: P-gp pump efflux