What risks do excipients or additives in horse ivermectin pose to humans if ingested?

1. Why veterinary formulations differ from human pills — and why that matters

The active molecule, ivermectin, is common to both human and animal products, but veterinary products for horses are formulated as concentrated pastes, injectables, or topical solutions designed for large animals and delivery routes that differ from human tablets, so a single dose intended for a 1,000‑lb horse can far exceed safe human exposure and produce overdose independent of excipient effects ^{[5] [6] [4]}.

2. Documented harms from people ingesting veterinary ivermectin — the clinical picture

Poison-control and clinical reports link ingestion of veterinary ivermectin to rapid-onset neurotoxicity in acute overdoses, hospitalizations, a minority of deaths, and a spectrum of effects including gastrointestinal upset, hypotension, confusion, hallucinations, seizures, coma and liver injury — consequences attributable to excessive ivermectin exposure that are reported repeatedly in poison center and case series data ^{[1] [2] [7] [4]}.

3. What the literature and public health notices say about excipients specifically

Public-health advisories and poison‑control guidance emphasize that veterinary products contain inactive ingredients not well studied in humans and that some of these excipients, or topical vehicle components if swallowed, “may be toxic” in large amounts or cause unexpected reactions; official warnings explicitly advise against ingesting formulations meant for skin application or animal use ^{[3] [2]}.

4. Mechanisms by which excipients can harm humans

Excipients can harm via three routes: they can be directly toxic when swallowed in concentrated quantities (for instance solvents, preservatives or stabilizers used in animal pastes or injectables), they can alter absorption of ivermectin increasing systemic exposure, or they can provoke allergic or idiosyncratic reactions because they were never tested for human pharmacology — these concerns are the basis for repeated FDA/CDC/poison‑center cautions ^{[3] [2] [8]}.

5. How to separate excipient risk from active‑ingredient overdose in real cases

Clinical series and poison‑center reports often implicate high ivermectin doses as the proximate cause of neurotoxicity, but many investigators and public‑health notices warn that excipients complicate interpretation: animal pastes are both highly concentrated and formulated with non‑human‑grade vehicles, so adverse events reported after ingestion are plausibly due to the active drug, the excipients, or a combination — published analyses do not always disentangle the two ^{[1] [3] [8]}.

6. Population vulnerabilities and interacting risks

Older adults, people on interacting medications (e.g., blood thinners), those with liver disease, and individuals with genetic susceptibilities may be at particular risk from either ivermectin overdosing or excipient toxicity; authorities specifically highlight the danger of drug‑drug interactions and genetic predispositions to adverse neurologic responses ^{[9] [3]}.

7. Limits of available reporting and practical takeaways

Available sources document harms and state that excipients “may be toxic” and are not evaluated for humans, but they generally do not catalogue specific excipient chemical identities or dose thresholds in veterinary ivermectin that cause human toxicity, so definitive attribution to a named additive in a given case is usually not possible from the published reports alone ^{[3] [6]}; nevertheless, regulators and clinicians uniformly recommend against ingesting veterinary formulations and advise seeking healthcare for any exposure ^{[2] [8]}.