How do recommended dosages of ivermectin vary by species and weight compared to human dosing guidelines?
Executive summary
Human oral ivermectin for parasitic infections is dosed by body weight at roughly 0.15–0.2 mg/kg (150–200 µg/kg) as a single dose for most indications; example tables translate that into 3 mg tablets for children and fixed tablet counts for adults (e.g., 3 mg for 15–25 kg; 12 mg for 65–84 kg) [1] [2]. Veterinary dosing varies widely by species and purpose: dogs receive tiny monthly heartworm-prevention doses (~0.0015–0.003 mg/lb), sheep liquid labels cite 200 µg/kg as a single drench, and equine products come in much larger total amounts intended for animals up to ~1,250 lb — making animal formulations and concentrations incompatible with human use [3] [4] [5].
1. Human dosing: weight-based, low microgram-per-kilogram regimens
Ivermectin prescribing for approved human parasitic diseases is calculated per kilogram of body weight: commonly 150 µg/kg for onchocerciasis and about 200 µg/kg for strongyloidiasis, often given as a single oral dose with repeat intervals determined by infection severity [6] [1]. U.S. product information and clinical references present the practical tablet-based equivalents — for example, pill-count tables and guidance that translate 150–200 µg/kg into 3 mg tablet counts [2] [6]. Authoritative clinical summaries (Mayo Clinic, Drugs.com, FDA labeling) converge on this weight-based, single-dose approach for humans [1] [2] [6].
2. Veterinary dosing: different species, different scales, different risks
Veterinary ivermectin products are formulated and labeled for animals and use doses that differ by species and purpose. Heartworm prevention in dogs uses extremely small monthly doses expressed in mg per pound (about 0.0015–0.003 mg/lb), which is orders of magnitude different from human therapeutic mg/kg dosing and comes with breed-specific toxicity concerns [3]. Product packaging for horses (paste syringes) is sized to treat animals up to ~1,250 lb and is not interchangeable with human tablet dosing; manufacturers explicitly label formulations “for horses only” [5]. Regulatory veterinary labels for some species (e.g., sheep) specify 200 µg/kg as a single drench for particular uses, again demonstrating how dose and route are set by species and indication [4].
3. Why you cannot equate tablet counts or animal paste to human mg/kg dose
Animal formulations differ in concentration, excipients, and intended routes; packaging and syringe rings on horse paste reflect total grams for large body mass and are not calibrated to safe human dosing [5]. Human dosing guidance is weight-based and given in micrograms per kilogram with tablet strengths (3 mg, 6 mg, 12 mg) intended for precise human dosing; substituting animal products risks overdosing, toxicity, and exposure to unapproved formulations [1] [2]. Veterinary guidance and human prescribing guidelines serve different regulatory, safety, and pharmacokinetic purposes and are not interchangeable [3] [4].
4. Safety and sensitivity: breed, species and human adverse reports
Dogs with certain genetic variants (MDR1 mutation, common in many herding breeds) can be extremely sensitive to ivermectin even at intended veterinary doses, illustrating that small numerical differences in dose or species-specific pharmacology produce large safety differences [3]. Human product labels and safety sections report that neurotoxicity and, rarely, severe events have appeared with recommended doses and overdoses, stressing the need to follow labeled human dosing [6]. Available sources do not mention using animal formulations safely in humans; they explicitly warn against cross-species use [5].
5. Practical comparisons: numbers that show the gap
Put numerically, a typical human therapeutic dose of 0.15–0.2 mg/kg equals 150–200 µg/kg; for a 70 kg adult that is roughly 10.5–14 mg once — commonly delivered as a 12 mg tablet or 3–4 tablets depending on the table in product guidance [1] [2]. By contrast, canine heartworm preventive monthly doses are roughly 0.0015–0.003 mg per pound, which for a 30-lb dog is only 0.045–0.09 mg monthly — thousands-fold smaller than human therapeutic single doses by body-mass scaling [3]. Equine pastes carry many grams intended for hundreds of kilograms, not human-scale microgram/kg dosing [5].
6. What the sources disagree on or leave out
Clinical sources agree on human weight-based dosing (150–200 µg/kg) and on the species-specific nature of veterinary dosing [1] [3] [4]. Sources diverge in some secondary summaries about exact mcg/kg targets (some list 150 µg/kg for certain infections, others 200 µg/kg for others) but that reflects indication-specific labeling rather than contradiction [6] [1] [7]. Available sources do not mention any safe protocol for converting animal ivermectin formulations into human doses; they instead warn against using animal products for people [5].
Bottom line: approved human ivermectin dosing is a tightly defined, weight-based microgram-per-kilogram regimen (usually 150–200 µg/kg) with tablet strengths and schedules tailored to infections; veterinary products use very different absolute amounts and concentrations matched to each species and indication, and animal products must not be used as substitutes for human prescriptions [1] [3] [5].