What are safe ivermectin formulations and dosing conversions between species?
Executive summary
Ivermectin formulations and doses differ by species and by indication; human-approved products are single-dose oral tablets or topical creams with standard weight‑based dosing (generally 150–200 µg/kg) while veterinary products span oral pastes, injectables, pour‑ons and long‑acting depots with much higher and variable dose regimens (for example, long‑acting ruminant products ~630 µg/kg) and are not interchangeable with human medicines [1] [2] [3] [4]. Using veterinary ivermectin in humans risks overdose, unpredictable bioavailability and toxic effects; regulatory labels and safety data document substantial formulation, dose and pharmacokinetic differences [5] [4].
1. Human formulations, approved dosing and practical limits
Human ivermectin is approved as oral tablets and topical creams with tightly defined, weight‑based dosing: common therapeutic guidance cites single oral doses around 150–200 micrograms per kilogram for many parasitic indications, with some protocols ranging 150–250 µg/kg and repeat dosing for scabies or strongyloidiasis as clinically indicated [1] [2] [6]. The FDA label documents human pharmacokinetic studies showing wide dose ranges tested (30–120 mg in trials) and warns that food—especially high‑fat meals—can markedly increase bioavailability (about 2.5‑fold for a 30 mg dose with a high‑fat meal) which can change exposure and risk [5]. Safety monitoring and established human dose ranges exist because the human formulations, strengths and approved indications are controlled and studied in clinical contexts [1] [6].
2. Veterinary formulations and much higher dosing regimens
Veterinary ivermectin exists in concentrated injectables, oral pastes (horse pastes with ~1.87% noted), pour‑ons and long‑acting depot products designed for ruminants; these products often use higher per‑kilogram dosing—classic veterinary therapeutic doses cited include around 150–200 µg/kg for some applications but many long‑acting or concentrated formulations use substantially higher doses (for example, certain long‑acting ruminant products around 630 µg/kg) and variable concentrations [7] [4] [3]. Formulation vehicles are tailored to species: slow‑release boluses and long‑acting injectables are deliberately designed to maintain plasma drug concentrations for weeks or months, a pharmacokinetic profile foreign to human oral tablet therapy [4] [8].
3. Why formulations are not interchangeable — pharmacology and regulation
Regulatory frameworks prioritize different endpoints for humans (safety, single‑dose exposures, drug interactions) versus animals (welfare, food‑safety residues, environmental impact), and those differences drive distinct formulations and dosing schedules; the veterinary and human products are expressly considered not interchangeable by regulators and academic reviews [4]. Inadvertent human exposure to veterinary formulations has produced treatment‑related neurologic and systemic signs in experimental or accidental contexts, and the FDA label highlights intoxication signs and altered pharmacokinetics at high doses [5]. The LD50 and species differences in blood‑brain barrier permeability further underscore risk: preclinical data show wide interspecies toxicity variation (example LD50 ranges cited for mice and dogs and extrapolated human‑equivalents in reviews) [9].
4. Practical guidance for dose conversion and clinical decisions
Established practice is to use approved human product dosing (µg/kg) for human patients and to rely on species‑specific veterinary dosing for animals; general veterinary references and product labels must be followed for each species because concentrations and recommended µg/kg can differ even within animal classes [10] [3]. Clinicians consider indications, repeat dosing intervals (e.g., repeat at 7–14 days for scabies/strongyloidiasis), patient comorbidities and interactions with CYP3A4 metabolizers when selecting human doses; veterinarians account for species‑specific pharmacokinetics and food‑animal residue rules [2] [6] [4]. Where reporting is silent, there is insufficient evidence to endorse ad hoc conversion formulas across species beyond following labeled µg/kg recommendations and professional guidance [4] [10].
5. Contested uses, misinformation risks and who benefits
Public controversy over off‑label use of veterinary ivermectin in humans highlights both medical risk and information asymmetries: advocates point to ivermectin’s broad antiparasitic history and some experimental studies, while regulators and clinicians stress lack of evidence for unapproved indications and documented toxicity risk when human dosing and formulations are ignored [8] [5]. Hidden agendas can include commercial interests in veterinary products or political promotion of unproven therapies; the authoritative sources consulted explicitly caution that veterinary and human products are not substitutes and recommend consultation with licensed clinicians or veterinarians [4] [10].