Are dosages and concentrations different between horse paste ivermectin and human ivermectin formulations?

1. Same molecule, different products: the basic split

The active ingredient, ivermectin, is chemically the same whether in a human tablet or a veterinary paste, but regulators and clinicians treat the products as distinct because the vehicle (tablet vs. paste vs. injectable), concentration, and intended dosing regimen differ; human prescriptions are typically single, weight‑based oral tablets, while animal products include concentrated pastes and liquids designed for much larger animals ^{[1] [5]}.

2. Concentration in horse paste: how concentrated are we talking?

US regulatory text and product data show common horse ivermectin pastes are formulated at about 1.87% ivermectin by weight (0.0187 mg per mg of paste), and a standard 6.08‑gram tube can therefore contain on the order of 100+ milligrams of active drug—orders of magnitude greater than a single human tablet—so small measurement errors can yield very large dose differences for people ^{[2] [6]}.

3. Human dosing: small, weight‑based, and medically supervised

Typical therapeutic dosing in humans for parasitic indications is around 0.2 mg/kg (200 micrograms per kilogram), a single weight‑based dose given under medical guidance; that yields doses in the milligram range for an adult (for example, roughly 12 mg for a 60 kg person), which is far smaller than many volumes of veterinary paste intended to treat hundreds of kilograms of animal weight ^{[7] [1]}.

4. Veterinary dosing targets much larger animals, not humans

Horse and livestock formulations are labeled to treat animals weighing hundreds to thousands of pounds, with horse doses sometimes reaching many hundreds of milligrams or even reported as high as 1,200 mg in practice—levels that are not comparable to recommended human doses and pose significant overdose risk if transposed without accurate conversion ^{[8] [7]}.

5. Formulation affects absorption and peak levels

Different vehicles (liquid drenches, pour‑ons, pastes) produce variable absorption and peak plasma concentrations; regulatory bioavailability studies showed liquid formulations can produce higher and earlier peak plasma ivermectin concentrations than paste formulations in horses, meaning identical amounts of active drug can behave differently depending on product form—another reason human clinicians rely on approved human formulations and dosing studies ^[3].

6. Purity, excipients, and safety labeling matter

Veterinary products are manufactured for animal use and may have different excipients, manufacturing standards, residue labeling, and contamination risk compared with human pharmaceuticals; public health authorities and veterinary experts have warned that animal formulations are not intended for human consumption because of these differences and the absence of human dosing/quality control labeling ^{[4] [1]}.

7. Real‑world consequences documented by poison centers

During waves of interest in repurposing ivermectin outside approved indications, poison control centers reported spikes in exposures linked to animal formulations—consistent with the concentration/dosing mismatch—and clinicians have documented toxicity from overdoses and inappropriate dosing, underscoring that different concentrations translate to real clinical hazards ^{[5] [1]}.

8. Alternative views and limits of reporting

Some sources emphasize that the core active drug is the same and equate basic pharmacology across species, noting that, in principle, dosing can be calculated by weight; however, public health guidance stresses that comparable pharmacokinetics, purity, and safety data for humans are secured only through approved human products and clinical dosing studies, and the provided reporting does not include exhaustive clinical trial comparisons of every formulation in humans ^{[1] [4]}.