Can horse dewormer formulations cause systemic toxicity if applied to human skin?
Executive summary
When humans use veterinary dewormers topically, the medical literature and public-health reporting show a narrow window: licensed human topical ivermectin formulations produce low systemic absorption, but the pharmacokinetics of many veterinary pastes/gels applied to human skin are unknown — and that uncertainty, plus differences in concentration and excipients, means systemic toxicity cannot be ruled out in all scenarios [1]. Public warnings about misuse — particularly during COVID-era self-medication — emphasize danger from incorrect products and routes, though most reporting focuses on oral ingestion of livestock products rather than documented transdermal poisoning [2] [3] [4].
1. Why the question matters: formulation, dose and route change the risk
Drugs behave differently when removed from the formulations and species they were designed for: a topical ivermectin cream approved for human use shows low systemic absorption in clinical studies, but the pharmacokinetics of veterinary ivermectin paste applied to human skin remain unstudied, so extrapolating safety is unsound [1]. Veterinary products often have higher concentrations, different solvents or “pastes” with excipients intended for equine use — all factors that change skin permeation and systemic exposure but are not characterized for humans in the literature provided [1] [5].
2. The evidence in humans: licensed topical ivermectin vs. unknown veterinary preparations
Controlled human data show that a 1% ivermectin cream results in low systemic absorption that plateaus by two weeks, supporting relative systemic safety for that specific human product [1]. By contrast, the same source explicitly notes that the pharmacokinetics of ivermectin 1.87% paste (a formulation found in horse dewormers) applied to human skin are unknown, and clinicians have warned against using unstudied veterinary formulations on people [1].
3. When skin can become a gateway: damaged skin, enhancers and large exposures
Skin integrity and formulation chemistry determine absorption: laboratory work and toxicology reviews show that some chemicals become more permeable with penetration enhancers and that higher local concentrations or damaged skin increase systemic uptake [6] [7]. The reporting assembled does not include human studies quantifying how much systemic ivermectin or other equine actives enter circulation after widespread dermal application of horse products, leaving a data gap on worst‑case exposures [6] [7].
4. Which veterinary ingredients raise special concerns
Public-health reporting has focused heavily on ivermectin because of misuse trends; authorities warned that livestock tablets and pastes are not approved for human use and can cause harm when taken orally, while clarifying that human-approved topical ivermectin differs from these veterinary products [2] [3] [4]. Other equine products—such as insecticidal shampoos containing pyrethrins and piperonyl butoxide—are described in product documents as poorly absorbed through intact skin, but those documents still advise washing exposed skin and note potential for adverse reactions, underlining product-specific risk profiles [8].
5. Real-world signals: misuse, misinformation and clinical caution
Dermatology case reports and reviews have documented patients misusing veterinary wormers for conditions like rosacea or scabies and have urged clinicians to counsel against unregulated use because of unknown absorption, the possibility of adverse events, and the risk of driving resistance in parasites — a non-toxicity harm that nonetheless flows from misuse [1]. News outlets and agencies repeatedly debunked claims that livestock ivermectin is a safe or effective COVID-19 treatment, focusing more on oral misuse than topical, but the repeated public-health advisories reflect a precautionary stance [2] [3] [4].
6. Bottom line and gaps: unlikely in some circumstances, unknown in others
The controlled-data answer is nuanced: licensed human topical ivermectin demonstrates low systemic absorption, suggesting systemic toxicity from similar, approved preparations is unlikely [1]. However, because veterinary formulations often differ in concentration and excipients and because there are no robust human pharmacokinetic studies for many horse pastes/gels applied to skin, systemic toxicity from dermal application of those veterinary products cannot be categorically excluded — especially if used over large areas, on broken skin, with penetration enhancers, or ingested instead of applied [1] [6] [7]. Public-health guidance and dermatology literature therefore advise against using veterinary dewormers on people and emphasize consulting clinicians and using approved human products [1] [2].