Characteristics of ivermectin toxicity in patients taking veterinary and human formulations for the prevention and treatment of COVID-19

US poison control centers reported increased cases of ivermectin toxicity during the COVID-19 pandemic. Previous descriptions of ivermectin toxicity have evaluated heterogeneous groups with a variety of ivermectin sources and dosage patterns. We sought to compare the clinical effects of ivermectin toxicity in patients taking human- vs. veterinary-formulations and acute- vs. chronic-ingestion patterns.We performed a retrospective analysis of cases from the Oregon Poison Center of ivermectin exposures for the prevention or treatment of COVID-19 that resulted in a healthcare visit over a 24-week period (14 August 2021 - 31 January 2022).We identified 37 cases of ivermectin toxicity. The median age of patients was 64 years, and most patients were male. The majority of patients were hospitalized (21) or treated in an emergency department (13). A minority were treated in an outpatient setting (3) and one patient died. Seventeen ingested veterinary formulations and fifteen ingested prescription tablets. Patients reported taking ivermectin for treatment (23) and prevention (14) of COVID-19. Clinical effects included neurotoxicity (30), gastrointestinal symptoms (14), and musculoskeletal complaints (7). Patients taking veterinary products took higher doses of ivermectin and had higher rates of altered mental status than those taking prescription tablets. Patients taking ivermectin chronically took smaller doses (daily dose of 13.5 mg) over a prolonged period (median 3.8 weeks) and developed toxicity that was milder than those with acute ingestions.Ivermectin toxicity developed in predominantly male patients >60 years old who ingested higher than recommended doses and developed neurologic symptoms. Patients who took a veterinary formulation of ivermectin ingested large single doses or large daily doses for several days and developed rapid onset of neurotoxicity. Patients with chronic toxicity developed milder symptoms and tended to take typical therapeutic doses, but continued therapy for weeks rather than days.