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What ivermectin doses are considered toxic and what symptoms indicate overdose?
Executive summary
Available reporting describes standard human ivermectin doses for parasitic infections as roughly 0.15–0.2 mg/kg (single dose) and notes much higher, off-label doses have caused toxicity; a U.S. case series linked doses such as 21 mg twice weekly and veterinary formulations to hospitalizations with gastrointestinal symptoms, confusion, ataxia, hypotension and seizures [1] [2]. Toxic signs reported across clinical reviews and case reports include nausea, vomiting, diarrhea, dizziness, weakness, seizure, encephalopathy/confusion, gait imbalance and visual symptoms — but precise human “toxic mg/kg” thresholds are not consistently defined in the available sources [3] [4] [2].
1. What standard human doses look like — the baseline doctors use
Clinical references state typical human dosing for approved parasitic uses is weight‑based and low: commonly 150–200 micrograms (0.15–0.2 mg) per kilogram as a single oral dose (each tablet often 3 mg), with repeat treatment schedules varying by condition [1] [5]. These are the dosages used in controlled therapy for conditions such as strongyloidiasis and onchocerciasis and form the “therapeutic window” clinicians expect to be safe under medical supervision [1] [5].
2. Where reports show people crossed from therapeutic into toxic use
Reporting from a New England Journal of Medicine letter and related accounts documents an outbreak of misuse during the COVID‑19 pandemic: people obtained veterinary formulations or took repeated human tablets, with one described regimen being 21 mg twice weekly for prevention and others using single large first‑time doses — 6 of 21 persons required hospitalization, 4 needed ICU care, and none died [2] [6]. Many of these toxic events developed quickly, often within hours after a large single dose [6].
3. Symptoms and the ivermectin toxidrome clinicians reported
Multiple clinical summaries and case reports describe a constellation of gastrointestinal and neurologic effects in overdose: nausea, vomiting, diarrhea and abdominal pain plus dizziness, confusion, decreased sensorium/encephalopathy, ataxia/gait imbalance, tremor or seizures, hypotension, visual disturbances and, in severe cases, coma [3] [4] [2]. The NEJM series specifically listed gastrointestinal distress in 4 patients, confusion in 3, ataxia and weakness in 2, hypotension in 2, and seizures in 1 [2].
4. How much is “too much”? — what sources say and what they do not
Available clinical sources do not converge on a single numeric human mg/kg threshold that universally produces toxicity; rather, toxicity reports often involve doses substantially above therapeutic dosing or use of veterinary formulations. Some case reports cite neurologic adverse effects above roughly 2 mg/kg/day in certain contexts, and one case series noted very large cumulative ingestions (for example, a reported 4 mg/kg in a 24‑hour period in a published case) tied to severe neurologic symptoms, but authoritative fixed cutoffs for humans are not uniformly stated in the provided material [4] [2]. Veterinary and animal literature reports much higher tolerated doses in livestock and pets, but animal tolerances do not translate directly to human safety [7] [8].
5. High‑risk situations that increase overdose likelihood or severity
Risk factors flagged by the literature include using veterinary products (higher, non‑human concentrations), repeated or cumulative dosing beyond single recommended human doses, and genetic or drug interactions that impair blood‑brain‑barrier efflux (mutations in ABCB1/MDR1 can permit central nervous system ivermectin accumulation) — such genetic susceptibility can cause severe neurologic effects even at lower doses [6] [4]. Concomitant depressant drugs (e.g., benzodiazepines) are suggested as potentially increasing CNS effects [9].
6. Clinical and public‑health implications: what to watch for and what sources recommend
Clinicians and poison centers advise that early signs — GI upset, dizziness, confusion, visual changes or gait disturbance — warrant urgent evaluation, especially if history reveals ingestion of many tablets or veterinary formulations; seizures, hypotension or progressive encephalopathy require immediate emergency care [2] [3]. Public guidance emphasizes not using ivermectin for COVID‑19 outside clinical trials and cautions against veterinary products for human use [3] [2].
7. Limitations, agenda and competing perspectives in the sources
Research and reviews presented here mix peer‑reviewed case series (NEJM) and clinical summaries or toxicology reports; animal pharmacology sources and veterinary safety data cite much higher tolerated doses in livestock, which could be misconstrued by non‑experts to imply human safety at those levels [8] [7]. Available sources do not offer a single consensus numeric toxic threshold in mg/kg for humans; they emphasize clinical presentation and context (dosage, formulation, host factors) rather than a precise cutoff [4] [2].
If you want, I can tabulate the reported therapeutic doses versus documented overdose regimens and their associated symptoms from these sources for quick reference.