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What are the known toxic or lethal dose ranges of ivermectin in humans and how reliable are those estimates?
Executive summary
Published estimates of ivermectin’s toxic and lethal doses in humans vary widely and are mostly inferred from animal LD50s, small human pharmacokinetic and high‑dose safety studies, case reports of misuse, and modelling papers (animal LD50 → human equivalent 2.02–43.24 mg/kg) [1][2]. Typical therapeutic human dosing is 0.15–0.20 mg/kg (150–200 μg/kg); safety data exist for single oral doses from about 6–30 mg in healthy volunteers and some repeated‑dose studies up to higher microgram/kg ranges, but reliable human LD50 values are not directly measured in people [1][3][4].
1. What the published numbers actually say: animal LD50s and “human‑equivalent” ranges
Toxicity work in animals gives concrete LD50s — for example, mice oral LD50 ≈ 25 mg/kg and dogs ≈ 80 mg/kg — and some summaries convert those figures into a broad human‑equivalent LD50 range of roughly 2.02–43.24 mg/kg (which for a 70 kg person would be ~141–3,026 mg) [1][2]. Those numbers appear in reference summaries and popular summaries but are extrapolations, not direct human measurements [1][2].
2. What human studies actually measured: tolerated high doses and pharmacokinetics
Clinical pharmacology studies directly tested single oral doses in healthy volunteers (commonly 6–30 mg and in some trials higher single doses up to 120 mg) and modeled plasma exposures; these studies report dose‑proportional increases in Cmax and AUC and half‑lives in the ~11–28+ hour range, informing safety margins but not an LD50 in humans [3][5]. Systematic reviews of high‑dose ivermectin summarize trials and PK models for doses from single 6–30 mg up to repeated microgram/kg regimens, indicating tolerability information but emphasizing that most safety confidence still rests on mass drug administration at ~150–200 μg/kg [4][3].
3. Clinical adverse events and case reports: what real‑world toxicity looks like
Reported severe toxicities in humans relate largely to neurologic and gastrointestinal effects (seizures, coma, vomiting, ataxia) after massive overdoses or ingestion of veterinary formulations; poison‑control data and news accounts linked spikes in misuse during the COVID‑19 pandemic to increased overdose presentations [6][7][8]. Individual fatality reports exist (for example, a Colorado coroner cited "probable ivermectin toxicity" after apparent ingestion of veterinary‑grade product), but formal toxicology confirmation and population‑level LD50 data in humans are usually lacking [7].
4. Why “human LD50” estimates are unreliable and vary so much
Human‑equivalent LD50s derived from animal LD50s assume interspecies scaling and ignore human genetic and physiological variability (notably P‑glycoprotein/MDR1 function at the blood‑brain barrier that affects CNS exposure), drug interactions, formulation differences (veterinary vs. human tablets), nutritional state (food increases bioavailability), and coexisting illnesses — all factors that alter toxicity risk [9][3][10]. Consequently, the wide range (2.02–43.24 mg/kg) reflects methodological extrapolation uncertainty rather than a precise human lethal threshold [1][2].
5. Therapeutic dose vs. putative “antiviral” or experimental doses — and safety implications
Standard approved human doses for parasitic indications are about 150–200 μg/kg; suggestions to use much higher doses (for example, up to several hundred micrograms/kg or mg/kg ranges to match in vitro antiviral concentrations) sparked concerns because the antiviral IC50s seen in cell culture would require plasma exposures not safely achievable in humans according to modelling and PK work [11][3]. In short: therapeutic doses are well under the extrapolated animal‑based lethal ranges, but attempting to reach in‑vitro antiviral concentrations would require doses that may approach toxic exposure ranges [11][3].
6. Competing perspectives and hidden agendas in the literature
Public health and regulatory sources (FDA, WHO summaries) emphasize that standard human dosing is safe and that animal formulations are inappropriate for people; media stories and clinical case reports emphasize harms from misuse and veterinary product ingestion [8][6]. Conversely, some advocacy or commercial webpages publish specific “human LD50” conversions (2.02–43.24 mg/kg) or claim higher tolerated doses for repurposing, often relying on extrapolations or selective citation; these sources can understate uncertainty in interspecies scaling and formulation differences [2][12].
7. Bottom line for clinicians, patients, and journalists
Available human data provide clear safe therapeutic ranges (≈150–200 μg/kg) and PK/safety data from single‑dose trials (6–30 mg and some dose‑escalation work) but do not yield a measured human LD50; animal‑derived “human‑equivalent LD50” estimates exist but are inherently uncertain and should not be treated as precise cutoffs [1][3][4]. Reports of severe toxicity or death largely involve high doses, veterinary formulations, or complicating factors; regulatory agencies warn against self‑medication and veterinary product use [8][6].
Limitations: direct human lethal‑dose experiments do not exist in the literature; extrapolations from animals and small human PK/safety trials form the evidence base [1][3]. Available sources do not mention a single, experimentally determined LD50 in humans.