Hello,
short answer is neurotoxicity.
Normeperidine (an active metabolite and CNS stimulant) may accumulate and precipitate anxiety, tremors, or seizures; risk increases with preexisting CNS or renal dysfunction, prolonged use (>48 hours), and cumulative dose (>600 mg/24 hours in adults). Oral meperidine should not be used since first-pass metabolism decreases efficacy while increasing normeperidine concentrations (APS 2016). Note: Naloxone does not reverse, and may even worsen, neurotoxicity.
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