What are the documented risks of using veterinary ivermectin formulations in humans and reported case outcomes?

1. Clinical picture: what toxicity looks like and who was affected

Case series and poison‑center analyses describe predominantly older, male patients who ingested doses higher than human recommendations and developed neurologic symptoms — altered mental status, ataxia, confusion, and in severe cases coma — often after large single doses or repeated dosing over days ^{[1] [6] [7]}. Emergency‑department and hospital presentations were common in these reports: of 37 Oregon Poison Center cases over a defined period most required ED visit or hospitalization and one death was reported in that cohort ^[1], while a NEJM case series reported six of 21 patients hospitalized for toxic effects ^[2].

2. Routes, formulations and dose: why animal products are riskier

Veterinary ivermectin comes in high‑concentration forms — injectable solutions for cattle, paste for horses, pour‑on concentrates — and dosing regimens intended for large animals; human ivermectin is available only as oral tablets with established dosing ^{[3] [4]}. Off‑label human use frequently involved ingesting veterinary pastes or solutions at doses that exceeded human therapeutic ranges, producing cumulative exposure and toxicity because some people gave repeat doses unlike the single‑use intent of many ivermectin indications ^{[4] [8]}.

3. Severe presentations and documented outcomes

Rare but severe outcomes have been documented: an unprecedented case of intravenous administration of veterinary ivermectin caused severe neurotoxicity with toxic serum levels and ICU care ^[9], and historical reports include coma and death after massive overdoses where the drug accumulates in the brain ^[7]. Contemporary surveillance showed increased poison‑control calls and clusters of adverse events during the COVID‑19 pandemic, including hospitalizations and at least one recorded fatality among small published cohorts ^{[10] [2] [1]}.

4. Mechanisms and compounding factors that increase danger

Ivermectin’s mechanism — potentiation of ligand‑gated chloride channels — is generally safe at human doses but can cause central nervous system depression if plasma and brain concentrations rise, especially with parenteral routes or very high oral intake; immature blood‑brain barriers or drug interactions could increase vulnerability ^{[9] [6] [7]}. Veterinary formulations may also contain excipients not tested in humans and concentration errors or mislabeling of dose when converting from animal to human use further raise risk ^{[3] [8]}.

5. Public‑health patterns, messages and motives

Public‑health alerts from FDA, CDC and state advisories documented surges in misuse and poison‑control calls tied to pandemic-driven demand, and institutions explicitly warned against substituting animal ivermectin for prescribed human products ^{[11] [3] [10]}. Some reporting and advocacy for ivermectin use relied on early lab data and small trials; critics and regulators argue that the push toward veterinary products was driven by misinformation, shortages and desperation rather than robust clinical evidence of benefit ^{[12] [2]}. Veterinary experts and academic centers emphasized the physiological and formulation differences between species to discourage off‑label animal use ^{[13] [5]}.

6. Limits of the evidence and open questions

Available documentation comes from poison‑center surveillance, retrospective case series and isolated case reports — strong for signalling harm but limited in scale and generalizability; long‑term toxicity data in humans after repeated overdoses are sparse and many studies caution that randomized trials do not support ivermectin for COVID‑19 clinical benefit ^{[1] [12] [6]}. Quantitative testing of veterinary products found ivermectin content often within label limits, but that does not eliminate danger from high intended animal dosages and improper human dosing regimens ^[8].