Which doses of ivermectin are considered toxic and how does toxicity differ by formulation (veterinary vs human)?
Executive summary
Therapeutic human ivermectin dosing is weight‑based and typically 150–400 micrograms/kg as a single dose for parasitic infections; usual regimens cited are 150–200 mcg/kg and mass‑treatment programs have used up to 0.4 mg/kg (400 mcg/kg) annually [1] [2]. Toxicity appears when doses exceed approved ranges — poison‑control guidance and toxicology reports flag problems above about 2 mg/kg and animal studies note toxic ranges of 5–15 mg/kg; veterinary formulations and high doses are associated with neurotoxicity (altered mental status, seizures, coma) and additional risks from untested excipients [3] [4] [5].
1. What doctors prescribe vs. what people take
Doctors prescribe ivermectin for human parasitic diseases using weight‑based oral tablets: common clinical guidance is about 150–200 micrograms per kilogram as a single dose, with some programs using 0.4 mg/kg as an annual mass‑treatment dose [1] [2]. Media and medical reviews emphasize that those prescribed doses are generally considered safe by clinicians, while higher or repeated unsupervised dosing is where harms concentrate [6] [7].
2. How toxicity shows up — the clinical picture
High ivermectin exposure causes central nervous system effects: blurred vision, confusion, seizures, ataxia, coma and, in severe cases, death. Case series and reporting during the COVID‑19 period documented neurotoxicity and altered mental status in people who took high doses, and major health bodies warn of ataxia, coma, and nervous‑system depression from overdose [6] [8] [3].
3. Numeric thresholds cited in toxicology and poison‑control work
Toxicology reports and prehospital guidance note that animal models often report toxicity at 5–15 mg/kg; poison‑control recommendations suggest hospital evaluation for exposures above about 2 mg/kg in people — substantially higher than therapeutic human dosing measured in micrograms per kilogram [3]. Systematic reviews note usual human therapeutic doses of 0.2–0.4 mg/kg are “extremely safe,” but also that higher doses used experimentally can produce serious adverse events [7].
4. Why veterinary formulations complicate risk
Veterinary ivermectin products are formulated at much higher concentrations and include excipients and possibly other actives not tested in humans. Clinical toxicology reviews and regulators explicitly warn that veterinary products differ from human tablets and that people using animal products reported neurotoxicity; the FDA and academic toxicologists advise never to use animal ivermectin for people because safety is unknown and can be dangerous [5] [9] [4].
5. Mechanisms and host vulnerability that alter toxicity
Part of ivermectin’s safety margin depends on P‑glycoprotein (MDR1/P‑gp) at the blood‑brain barrier, which pumps the drug out of the CNS. Genetic or pharmacologic impairments of this transporter—known in some animals and relevant in humans—can increase CNS accumulation and risk of neurotoxicity [10] [5].
6. Differences by formulation beyond dose — excipients and routes
Veterinary ivermectin comes as high‑concentration pour‑ons, injectables, pastes and feeds designed for large animals; these routes and inactive ingredients can produce unpredictable absorption and additional toxicities in people. Several sources flag untested excipients (for example thiabendazole in some horse products) that can themselves be harmful if ingested by humans [5] [11] [12].
7. What the scientific literature and clinicians say about high‑dose experiments
Laboratory antiviral or anticancer effects were observed in vitro at concentrations far above approved human exposures; commentators and oncologists warn that doses producing those preclinical effects would likely be toxic in humans and that clinical evidence of benefit is lacking [13] [14]. Systematic reviews acknowledge usual doses (0.2–0.4 mg/kg) are safe but document serious adverse events when higher or prolonged regimens were tried [7].
8. Practical takeaways and reporting limitations
Available clinical and regulatory reports establish a clear gap between therapeutic human doses (micrograms/kg) and doses associated with toxicity (milligrams/kg in toxicology reports and animal models), and they document greater risk when people use veterinary products [1] [3] [5]. Limitations: the provided sources do not give a single universally accepted human “LD50” or an exact microgram/kg cutoff at which every person will become symptomatic; individual susceptibility (P‑gp status, age, comorbidities, co‑medications), formulation ingredients, and route of exposure alter risk and are noted in the literature [10] [7].
If you want, I can summarize these numeric ranges into a concise chart (therapeutic band vs. reported toxic ranges vs. veterinary product concentrations) using only the values cited above.